@Steve Christy Thank you for being patient while working on this issue.
You can use the migration service and use VM as a source and enter the flexible server credentials. It has worked for many customers.
OR
Move PostgreSQL Flexible server from source to target subscription (which did not move VNET to target subscription as expected). Then do PITR of the server to move to the target subscription and use VNET from the target subscription during restore.
Regards,
Oury