So I think I can reproduce this in a basic devstack env with a simple 2 process uwsgi config and enable-threads = true:

$ cat /<email address hidden>

[Unit]
Description = Devstack <email address hidden>

[Service]
RestartForceExitStatus = 100
NotifyAccess = all
Restart = always
KillMode = process
Type = notify
ExecReload = /usr/bin/kill -HUP $MAINPID
ExecStart = /usr/local/bin/uwsgi --procname-prefix nova-api --ini /etc/nova/nova-api-uwsgi.ini
User = stack
SyslogIdentifier = <email address hidden>

[Install]
WantedBy = multi-user.target

$ cat /etc/nova/nova-api-uwsgi.ini

[uwsgi]
chmod-socket = 666
socket = /var/run/uwsgi/nova-api-wsgi.socket
lazy-apps = true
add-header = Connection: close
buffer-size = 65535
hook-master-start = unix_signal:15 gracefully_kill_them_all
thunder-lock = true
plugins = http,python3
enable-threads = true
worker-reload-mercy = 90
exit-on-reload = false
die-on-term = true
master = true
processes = 2
wsgi-file = /usr/local/bin/nova-api-wsgi

$ for i in $(seq 1 9); do openstack volume create --size 1 test-$i ; done
$ for i in $(seq 1 9); do openstack server add volume test test-$i & done
[1] 888087
[2] 888088
[3] 888089
[4] 888090
[5] 888091
[6] 888092
[7] 888093
[8] 888094
[9] 888095
[5]   Done                    openstack server add volume test test-$i
[2]   Done                    openstack server add volume test test-$i
[4]   Done                    openstack server add volume test test-$i
[8]-  Done                    openstack server add volume test test-$i
[9]+  Done                    openstack server add volume test test-$i
[1]   Done                    openstack server add volume test test-$i
[3]   Done                    openstack server add volume test test-$i
[6]-  Done                    openstack server add volume test test-$i
[7]+  Done                    openstack server add volume test test-$i
$ openstack server event list test
+------------------------------------------+--------------------------------------+---------------+----------------------------+
| Request ID                               | Server ID                            | Action        | Start Time                 |
+------------------------------------------+--------------------------------------+---------------+----------------------------+
| req-df477597-befd-4122-bc3d-d54e9b0daaae | 589295a7-14bf-40cb-8f05-1b225448002e | attach_volume | 2021-06-02T09:00:10.000000 |
| req-17e8c4af-a435-4d91-854a-9308a19284d3 | 589295a7-14bf-40cb-8f05-1b225448002e | attach_volume | 2021-06-02T09:00:10.000000 |
| req-336f0d30-716a-4294-90d9-33869192e961 | 589295a7-14bf-40cb-8f05-1b225448002e | attach_volume | 2021-06-02T09:00:03.000000 |
| req-7b9d3b8a-e94d-446e-890f-216cc8f8d6c3 | 589295a7-14bf-40cb-8f05-1b225448002e | attach_volume | 2021-06-02T09:00:03.000000 |
| req-a2e80b18-113f-436f-8fa8-4c2373b9d831 | 589295a7-14bf-40cb-8f05-1b225448002e | attach_volume | 2021-06-02T08:59:57.000000 |
| req-8d24416c-b80d-4230-8cf4-9a979690e7ff | 589295a7-14bf-40cb-8f05-1b225448002e | attach_volume | 2021-06-02T08:59:56.000000 |
| req-0d91f187-e4c7-4469-bd66-4880add4b2df | 589295a7-14bf-40cb-8f05-1b225448002e | attach_volume | 2021-06-02T08:59:53.000000 |
| req-c4b0e772-a062-455e-abc2-201e797b1522 | 589295a7-14bf-40cb-8f05-1b225448002e | attach_volume | 2021-06-02T08:59:52.000000 |
| req-6bfccd1e-411c-4314-9ab0-9d3130e36866 | 589295a7-14bf-40cb-8f05-1b225448002e | attach_volume | 2021-06-02T08:59:52.000000 |
| req-8995e5f9-f30e-4891-8c34-20538955f961 | 589295a7-14bf-40cb-8f05-1b225448002e | reboot        | 2021-05-27T17:35:37.000000 |
| req-78e538e8-7cc2-4142-88ee-0c0428f313b5 | 589295a7-14bf-40cb-8f05-1b225448002e | attach_volume | 2021-05-27T17:33:01.000000 |
| req-82baacc7-9ea1-40df-b2b3-78f755723d5e | 589295a7-14bf-40cb-8f05-1b225448002e | create        | 2021-05-27T17:32:49.000000 |
+------------------------------------------+--------------------------------------+---------------+----------------------------+

Capture the request being accepted and then returning after the RPC call to the compute (please forgive the horrible one liner):

$ for req in $(openstack server event list test | grep attach_volume | awk '{ print $2 }'); do journalctl -u devstack@n-api -g $req -q | egrep '(nova.api.openstack.wsgi|nova.api.openstack.requestlog)' ; done

Jun 02 04:59:52 controller.example.com <email address hidden>[140959]: DEBUG nova.api.openstack.wsgi [None req-6bfccd1e-411c-4314-9ab0-9d3130e36866 admin admin] Action: 'create', calling method: <bound method VolumeAttachmentController.create of <nova.api.openstack.compute.volumes.VolumeAttachmentController object at 0x7f616cc57550>>, body: {"volumeAttachment": {"volumeId": "93f7e994-ab2e-424f-9257-c17f94e87c15"}} {{(pid=140959) _process_stack /opt/stack/nova/nova/api/openstack/wsgi.py:511}}
Jun 02 04:59:52 controller.example.com <email address hidden>[140959]: INFO nova.api.openstack.requestlog [None req-6bfccd1e-411c-4314-9ab0-9d3130e36866 admin admin] 192.168.122.178 "POST /compute/v2.1/servers/589295a7-14bf-40cb-8f05-1b225448002e/os-volume_attachments" status: 200 len: 194 microversion: 2.1 time: 0.238063

Jun 02 04:59:52 controller.example.com <email address hidden>[140959]: DEBUG nova.api.openstack.wsgi [None req-c4b0e772-a062-455e-abc2-201e797b1522 admin admin] Action: 'create', calling method: <bound method VolumeAttachmentController.create of <nova.api.openstack.compute.volumes.VolumeAttachmentController object at 0x7f616cc57550>>, body: {"volumeAttachment": {"volumeId": "89f048cf-f1a9-4153-9283-4f8eb9b7dd67"}} {{(pid=140959) _process_stack /opt/stack/nova/nova/api/openstack/wsgi.py:511}}
Jun 02 04:59:56 controller.example.com <email address hidden>[140959]: INFO nova.api.openstack.requestlog [None req-c4b0e772-a062-455e-abc2-201e797b1522 admin admin] 192.168.122.178 "POST /compute/v2.1/servers/589295a7-14bf-40cb-8f05-1b225448002e/os-volume_attachments" status: 200 len: 194 microversion: 2.1 time: 3.524594

Jun 02 04:59:53 controller.example.com <email address hidden>[140958]: DEBUG nova.api.openstack.wsgi [None req-0d91f187-e4c7-4469-bd66-4880add4b2df admin admin] Action: 'create', calling method: <bound method VolumeAttachmentController.create of <nova.api.openstack.compute.volumes.VolumeAttachmentController object at 0x7f616ccb8ee0>>, body: {"volumeAttachment": {"volumeId": "cadbab27-fa90-4139-9ece-d612b8e6c081"}} {{(pid=140958) _process_stack /opt/stack/nova/nova/api/openstack/wsgi.py:511}}
Jun 02 04:59:56 controller.example.com <email address hidden>[140958]: INFO nova.api.openstack.requestlog [None req-0d91f187-e4c7-4469-bd66-4880add4b2df admin admin] 192.168.122.178 "POST /compute/v2.1/servers/589295a7-14bf-40cb-8f05-1b225448002e/os-volume_attachments" status: 200 len: 194 microversion: 2.1 time: 2.918409

Jun 02 04:59:56 controller.example.com <email address hidden>[140959]: DEBUG nova.api.openstack.wsgi [None req-8d24416c-b80d-4230-8cf4-9a979690e7ff admin admin] Action: 'create', calling method: <bound method VolumeAttachmentController.create of <nova.api.openstack.compute.volumes.VolumeAttachmentController object at 0x7f616cc57550>>, body: {"volumeAttachment": {"volumeId": "aa2c9bea-cd33-4c70-9689-b743329725b9"}} {{(pid=140959) _process_stack /opt/stack/nova/nova/api/openstack/wsgi.py:511}}
Jun 02 05:00:03 controller.example.com <email address hidden>[140959]: INFO nova.api.openstack.requestlog [None req-8d24416c-b80d-4230-8cf4-9a979690e7ff admin admin] 192.168.122.178 "POST /compute/v2.1/servers/589295a7-14bf-40cb-8f05-1b225448002e/os-volume_attachments" status: 200 len: 194 microversion: 2.1 time: 6.806390

Jun 02 04:59:57 controller.example.com <email address hidden>[140958]: DEBUG nova.api.openstack.wsgi [None req-a2e80b18-113f-436f-8fa8-4c2373b9d831 admin admin] Action: 'create', calling method: <bound method VolumeAttachmentController.create of <nova.api.openstack.compute.volumes.VolumeAttachmentController object at 0x7f616ccb8ee0>>, body: {"volumeAttachment": {"volumeId": "c4b0ab74-5ada-43ec-b48b-cebdc286d273"}} {{(pid=140958) _process_stack /opt/stack/nova/nova/api/openstack/wsgi.py:511}}
Jun 02 05:00:03 controller.example.com <email address hidden>[140958]: INFO nova.api.openstack.requestlog [None req-a2e80b18-113f-436f-8fa8-4c2373b9d831 admin admin] 192.168.122.178 "POST /compute/v2.1/servers/589295a7-14bf-40cb-8f05-1b225448002e/os-volume_attachments" status: 200 len: 194 microversion: 2.1 time: 6.520442

Jun 02 05:00:03 controller.example.com <email address hidden>[140958]: DEBUG nova.api.openstack.wsgi [None req-336f0d30-716a-4294-90d9-33869192e961 admin admin] Action: 'create', calling method: <bound method VolumeAttachmentController.create of <nova.api.openstack.compute.volumes.VolumeAttachmentController object at 0x7f616ccb8ee0>>, body: {"volumeAttachment": {"volumeId": "6c8e6d76-1a02-4396-9f60-f702710dbab2"}} {{(pid=140958) _process_stack /opt/stack/nova/nova/api/openstack/wsgi.py:511}}
Jun 02 05:00:10 controller.example.com <email address hidden>[140958]: INFO nova.api.openstack.requestlog [None req-336f0d30-716a-4294-90d9-33869192e961 admin admin] 192.168.122.178 "POST /compute/v2.1/servers/589295a7-14bf-40cb-8f05-1b225448002e/os-volume_attachments" status: 200 len: 194 microversion: 2.1 time: 7.171922

Jun 02 05:00:03 controller.example.com <email address hidden>[140959]: DEBUG nova.api.openstack.wsgi [None req-7b9d3b8a-e94d-446e-890f-216cc8f8d6c3 admin admin] Action: 'create', calling method: <bound method VolumeAttachmentController.create of <nova.api.openstack.compute.volumes.VolumeAttachmentController object at 0x7f616cc57550>>, body: {"volumeAttachment": {"volumeId": "dac5e1e5-9f0f-4b53-86d5-0e938e24168d"}} {{(pid=140959) _process_stack /opt/stack/nova/nova/api/openstack/wsgi.py:511}}
Jun 02 05:00:10 controller.example.com <email address hidden>[140959]: INFO nova.api.openstack.requestlog [None req-7b9d3b8a-e94d-446e-890f-216cc8f8d6c3 admin admin] 192.168.122.178 "POST /compute/v2.1/servers/589295a7-14bf-40cb-8f05-1b225448002e/os-volume_attachments" status: 200 len: 194 microversion: 2.1 time: 7.168875

Jun 02 05:00:10 controller.example.com <email address hidden>[140958]: DEBUG nova.api.openstack.wsgi [None req-df477597-befd-4122-bc3d-d54e9b0daaae admin admin] Action: 'create', calling method: <bound method VolumeAttachmentController.create of <nova.api.openstack.compute.volumes.VolumeAttachmentController object at 0x7f616ccb8ee0>>, body: {"volumeAttachment": {"volumeId": "ad2e3f58-b771-42e4-97ab-a4e2921a0e80"}} {{(pid=140958) _process_stack /opt/stack/nova/nova/api/openstack/wsgi.py:511}}
Jun 02 05:00:17 controller.example.com <email address hidden>[140958]: INFO nova.api.openstack.requestlog [None req-df477597-befd-4122-bc3d-d54e9b0daaae admin admin] 192.168.122.178 "POST /compute/v2.1/servers/589295a7-14bf-40cb-8f05-1b225448002e/os-volume_attachments" status: 200 len: 194 microversion: 2.1 time: 7.131011

Jun 02 05:00:10 controller.example.com <email address hidden>[140959]: DEBUG nova.api.openstack.wsgi [None req-17e8c4af-a435-4d91-854a-9308a19284d3 admin admin] Action: 'create', calling method: <bound method VolumeAttachmentController.create of <nova.api.openstack.compute.volumes.VolumeAttachmentController object at 0x7f616cc57550>>, body: {"volumeAttachment": {"volumeId": "f009530c-44b8-4116-a78e-4fa66936b841"}} {{(pid=140959) _process_stack /opt/stack/nova/nova/api/openstack/wsgi.py:511}}
Jun 02 05:00:17 controller.example.com <email address hidden>[140959]: INFO nova.api.openstack.requestlog [None req-17e8c4af-a435-4d91-854a-9308a19284d3 admin admin] 192.168.122.178 "POST /compute/v2.1/servers/589295a7-14bf-40cb-8f05-1b225448002e/os-volume_attachments" status: 200 len: 194 microversion: 2.1 time: 7.124412


I'll play around with this some more to confirm.