@Yvonne Ding, I can't reproduce this issue in my environment. Can you share more information about your test? For example, network configuration, test app, etc.

Below is my configuration:

1. I deployed a multi (2+2) with stx.4.0-20200717T143848Z.
2. vm network 
| mtu                       | 1500                                                                                                                                                                           |
| name                      | public-net0                                                                                                                                                                    |
| port_security_enabled     | True                                                                                                                                                                           |
| project_id                | 3f7e6801f7b140f49b060952e32eba83                                                                                                                                               |
| provider:network_type     | vlan                                                                                                                                                                           |
| provider:physical_network | physnet0                                                                                                                                                                       |
| provider:segmentation_id  | 400

3. subnet
controller-0:~$ openstack subnet show public-subnet0
| allocation_pools  | 192.168.101.2-192.168.101.254                                                                                                                                                  |
| cidr              | 192.168.101.0/24                                                                                                                                                               |
| created_at        | 2020-07-24T03:09:22Z                                                                                                                                                           |
| description       |                                                                                                                                                                                |
| dns_nameservers   |                                                                                                                                                                                |
| enable_dhcp       | True                                                                                                                                                                           |
| gateway_ip        | 192.168.101.1                                                                                                                                                                  |
| host_routes       |                                                                                                                                                                                |
| id                | 5d19f453-ff33-42c8-a067-c2f6059c3629                                                                                                                                           |
| ip_version        | 4                                                                                                                                                                              |
| ipv6_address_mode | None                                                                                                                                                                           |
| ipv6_ra_mode      | None                                                                                                                                                                           |
| location          | cloud='openstack_helm', project.domain_id=, project.domain_name='default', project.id='3f7e6801f7b140f49b060952e32eba83', project.name='admin', region_name='RegionOne', zone= |
| name              | public-subnet0                                                                                                                                                                 |

4. vm image: CentOS-7-x86_64-GenericCloud.qcow2

5. The mtu of the two vms are both 1500
[root@vm2 ~]# ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.101.34  netmask 255.255.255.0  broadcast 192.168.101.255

[root@vm1 ~]# ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.101.21  netmask 255.255.255.0  broadcast 192.168.101.255

5. test app: I wrote a simple udp client/server with python.

6. I tried payload size from 1400 - 2000. No packet loss found.
[root@vm2 ~]# ./client.py --padding 2000
start to send package to 192.168.101.21:12345
0: send 2025 bytes to ('192.168.101.21', 12345)
1: send 2025 bytes to ('192.168.101.21', 12345)
2: send 2025 bytes to ('192.168.101.21', 12345)
3: send 2025 bytes to ('192.168.101.21', 12345)
4: send 2025 bytes to ('192.168.101.21', 12345)

[root@vm2 ~]# ./client.py --padding 1400
start to send package to 192.168.101.21:12345
0: send 1425 bytes to ('192.168.101.21', 12345)
1: send 1425 bytes to ('192.168.101.21', 12345)
2: send 1425 bytes to ('192.168.101.21', 12345)
3: send 1425 bytes to ('192.168.101.21', 12345)
4: send 1425 bytes to ('192.168.101.21', 12345)

[root@vm1 ~]# ./server.py
start to wait for message on 0.0.0.0:12345
receive message from ('192.168.101.34', 38665): 0 len=2025
receive message from ('192.168.101.34', 38665): 1 len=2025
receive message from ('192.168.101.34', 38665): 2 len=2025
receive message from ('192.168.101.34', 38665): 3 len=2025
receive message from ('192.168.101.34', 38665): 4 len=2025
receive message from ('192.168.101.34', 48803): 0 len=1425
receive message from ('192.168.101.34', 48803): 1 len=1425
receive message from ('192.168.101.34', 48803): 2 len=1425
receive message from ('192.168.101.34', 48803): 3 len=1425
receive message from ('192.168.101.34', 48803): 4 len=1425