TLDR; Azure OS Family 5 requires Remote Desktop passwords >= 10 characters. Anything less will cause your login to fail, repeatedly requesting that you re-enter your password.
I ran into an issue when upgrading an Azure application from OS Family 4 to OS Family 5. We have configured RDP for our development deployments. As part of that deployment, we had configured special passwords for each environment. Those passwords had a strong enough length when we added them a few years ago: 8 and 9 characters. OS Family 5 (Windows Server 2016) requires that the passwords are at least 10 characters long.
As a result, we found that the deployment went fine (no errors reported) but that we simply couldn’t log in post upgrade. Looking on the portal, we noted that one has to have a password of at least 10 characters to add Remote Desktop from the portal. We counted the characters in our passwords, adjusted lengths, and found we could login again.