@jw Thanks for providing additional information
I just tried deploying Postgres flex 11 with Public and Private region West Europ and I did not face any issues.I was able to deploy it successfully
As an alternative can you try deploying using CLI and let me know if you are still facing the same issue
https://learn.microsoft.com/en-us/azure/postgresql/flexible-server/how-to-manage-virtual-network-cli
Regards
Geetha