Azure currently does not support in-place restores using the Point-in-Time Recovery feature of Azure PSQL Flexible server, so you have to always initially create a standalone flexible server resource. There is no one-click way to get Terraform to recognize this newly restored server without adding it to your Terraform code first and then using Terraform import commands. Deleting the original server will not address any state management issues.

The other alternative outside of imports would be to add some additional complexity to your Terraform code to conditionally create the restored server and manage in-line import blocks. Given that approach requires a bit of a deep dive I wrote an article here for further review.