To Reproduce Steps to reproduce the behavior:

1. Create a new project 'testproject', a new user on 'testproject' with a role member, create aswel a network and subnet for testing a VM
2. from admin project, create a SG and share it for project 'testproject'
3. from 'testproject' using the CLI, create a VM and add the shared SG
4. the shared SG is shown for the VM
5. login to horizon and add a new SG for the VM; you can see that the shared SG is not listed
6. after you saved the VM SG change, the shared SG get removed from VM, you can confirm this checking by CLI

Expected behavior

• horizon shows and handle shared SG

Bug impact

• shared SG are not handled in horizon while they works at CLI. They don't show out
• When CU has a VM with a shared SG and add another SG by horizon, the shared SG get removed from VM
• removing SG causes VMs disruptions

Known workaround

• this doesn't happen with the CLI

This seems the same upstream bug https://bugs.launchpad.net/horizon/+bug/1905587

reproduced on RHOSP 17.1.9