We had very similar symptoms (the user always gets a username/password prompt when trying to print) but it seems for us PeerCred was not working properly. I tried compiling the source tree from cosmic and it worked correctly. I then took a diff between that and the bionic package and the cups/auth.c has 3 different lines. Applying this patch to the 2.2.7 source tree fixes the issue at least for us.
We had very similar symptoms (the user always gets a username/password prompt when trying to print) but it seems for us PeerCred was not working properly. I tried compiling the source tree from cosmic and it worked correctly. I then took a diff between that and the bionic package and the cups/auth.c has 3 different lines. Applying this patch to the 2.2.7 source tree fixes the issue at least for us.