After the switches at work got segmented over the weekend, one of our Samba 4.15 filesservers had trouble with 'getent passwd username' not returning any user information, even though winbind -u was returning a list of users. 'wbinfo --user-info username' was returning error about not being connected to a DC.
This was after the network was restarted and I could log into machines again.
Had to wipe samba (/var/lib/samba contents), and reinstall and rejoin the Domain which was easy becuase just implementing Ansible devops at work.
This should not be happening though. winbind AD connection state machine should be retrying and not getting stuck in a failed connection state like this.