@jamesh, thank you very much! I have tested your core20 test package now and it works!
It took me some time until getting everything neutralized and reproducing the original problem. First, with your test core20 installed one cannot build Snaps. I had to go back to the originals to build a modified CUPS (workaround removed) for reproducing the problem. I also had to snap clean the build and to purge the installed CUPS Snap to get really rid of the workaround. But finally I found back to the problem appearing, installed your corrected core20 and it works!
So from my side and the point of view that yiour changes solve the problem the new core20 is approved.