I seem to be having this problem too.
I had another network problem on my gateway after the update, which was resolved by editing /etc/network/interfaces (whonixcheck fail: tor.pid does not exist after upgrade from Whonix 11 to Whonix 12 - #12 by Patrick)
Everything seemed to be working for a day or two after that. Then I’m not sure if it was after a reset or when it happened but I have lost network connection on my Workstation. Gateway still has a connection. I did not touch Workstation networking.
edit: To clarify whonixcheck on Gateway comes back all good. Whonixcheck on workstation “ERROR: Tor Boostrap Result: Tor’s Control Port could not be reached!”