Yes it is a bug, i have tested on my system. below are the reproducing steps :

Precondition:
create two instance and attach them with a multiattach volume.
there instances id are :  c78bdc4b-ebb0-41d2-a435-6f56791a9604, 0efcd163-13ab-4b70-9449-ab89301be1cf  

1. delete first attachment : 

[root@openstackq ~(keystone_admin)]# nova volume-detach c78bdc4b-ebb0-41d2-a435-6f56791a9604 4361ce05-e325-40c3-8b2c-5bcaeedf4260

2. now, delete second attachment :

[root@openstackq ~(keystone_admin)]# nova volume-detach 0efcd163-13ab-4b70-9449-ab89301be1cf 4361ce05-e325-40c3-8b2c-5bcaeedf4260


Second step executed without any message.

however, if we check output of cinder list, volume's status is shown as a "deattaching". but after few seconds it changes from "deattaching" to "in-use".

3.

[root@openstackq ~(keystone_admin)]# cinder list
+--------------------------------------+-----------+-----------+------+-------------+----------+---------------------------------------------------------------------------+
| ID                                   | Status    | Name      | Size | Volume Type | Bootable | Attached to                                                               |
+--------------------------------------+-----------+-----------+------+-------------+----------+---------------------------------------------------------------------------+
| 4361ce05-e325-40c3-8b2c-5bcaeedf4260 | detaching | -         | 1    | multiattach | false    | 0efcd163-13ab-4b70-9449-ab89301be1cf                                      |
| 946009e9-a08f-46d4-88f8-c302d2ec9d75 | in-use    | sharedvol | 1    | multiattach | false    | 2bf44c23-87b4-4d84-81b6-bc2b1f792d12,c79ad598-be11-490c-8fd0-58c6d72bbe4f |
+--------------------------------------+-----------+-----------+------+-------------+----------+---------------------------------------------------------------------------+
[root@openstackq ~(keystone_admin)]# cinder list
+--------------------------------------+-----------+-----------+------+-------------+----------+---------------------------------------------------------------------------+
| ID                                   | Status    | Name      | Size | Volume Type | Bootable | Attached to                                                               |
+--------------------------------------+-----------+-----------+------+-------------+----------+---------------------------------------------------------------------------+
| 4361ce05-e325-40c3-8b2c-5bcaeedf4260 | detaching | -         | 1    | multiattach | false    | 0efcd163-13ab-4b70-9449-ab89301be1cf                                      |
| 946009e9-a08f-46d4-88f8-c302d2ec9d75 | in-use    | sharedvol | 1    | multiattach | false    | 2bf44c23-87b4-4d84-81b6-bc2b1f792d12,c79ad598-be11-490c-8fd0-58c6d72bbe4f |
+--------------------------------------+-----------+-----------+------+-------------+----------+---------------------------------------------------------------------------+
[root@openstackq ~(keystone_admin)]# cinder list
+--------------------------------------+-----------+-----------+------+-------------+----------+---------------------------------------------------------------------------+
| ID                                   | Status    | Name      | Size | Volume Type | Bootable | Attached to                                                               |
+--------------------------------------+-----------+-----------+------+-------------+----------+---------------------------------------------------------------------------+
| 4361ce05-e325-40c3-8b2c-5bcaeedf4260 | detaching | -         | 1    | multiattach | false    | 0efcd163-13ab-4b70-9449-ab89301be1cf                                      |
| 946009e9-a08f-46d4-88f8-c302d2ec9d75 | in-use    | sharedvol | 1    | multiattach | false    | 2bf44c23-87b4-4d84-81b6-bc2b1f792d12,c79ad598-be11-490c-8fd0-58c6d72bbe4f |
+--------------------------------------+-----------+-----------+------+-------------+----------+---------------------------------------------------------------------------+
[root@openstackq ~(keystone_admin)]# cinder list
+--------------------------------------+-----------+-----------+------+-------------+----------+---------------------------------------------------------------------------+
| ID                                   | Status    | Name      | Size | Volume Type | Bootable | Attached to                                                               |
+--------------------------------------+-----------+-----------+------+-------------+----------+---------------------------------------------------------------------------+
| 4361ce05-e325-40c3-8b2c-5bcaeedf4260 | detaching | -         | 1    | multiattach | false    | 0efcd163-13ab-4b70-9449-ab89301be1cf                                      |
| 946009e9-a08f-46d4-88f8-c302d2ec9d75 | in-use    | sharedvol | 1    | multiattach | false    | 2bf44c23-87b4-4d84-81b6-bc2b1f792d12,c79ad598-be11-490c-8fd0-58c6d72bbe4f |
+--------------------------------------+-----------+-----------+------+-------------+----------+---------------------------------------------------------------------------+
[root@openstackq ~(keystone_admin)]# cinder list
+--------------------------------------+-----------+-----------+------+-------------+----------+---------------------------------------------------------------------------+
| ID                                   | Status    | Name      | Size | Volume Type | Bootable | Attached to                                                               |
+--------------------------------------+-----------+-----------+------+-------------+----------+---------------------------------------------------------------------------+
| 4361ce05-e325-40c3-8b2c-5bcaeedf4260 | detaching | -         | 1    | multiattach | false    | 0efcd163-13ab-4b70-9449-ab89301be1cf                                      |
| 946009e9-a08f-46d4-88f8-c302d2ec9d75 | in-use    | sharedvol | 1    | multiattach | false    | 2bf44c23-87b4-4d84-81b6-bc2b1f792d12,c79ad598-be11-490c-8fd0-58c6d72bbe4f |
+--------------------------------------+-----------+-----------+------+-------------+----------+---------------------------------------------------------------------------+
[root@openstackq ~(keystone_admin)]# cinder list
+--------------------------------------+--------+-----------+------+-------------+----------+---------------------------------------------------------------------------+
| ID                                   | Status | Name      | Size | Volume Type | Bootable | Attached to                                                               |
+--------------------------------------+--------+-----------+------+-------------+----------+---------------------------------------------------------------------------+
| 4361ce05-e325-40c3-8b2c-5bcaeedf4260 | in-use | -         | 1    | multiattach | false    | 0efcd163-13ab-4b70-9449-ab89301be1cf                                      |
| 946009e9-a08f-46d4-88f8-c302d2ec9d75 | in-use | sharedvol | 1    | multiattach | false    | 2bf44c23-87b4-4d84-81b6-bc2b1f792d12,c79ad598-be11-490c-8fd0-58c6d72bbe4f |
+--------------------------------------+--------+-----------+------+-------------+----------+---------------------------------------------------------------------------+