Here is an update after a few more tests: It seems that adding new files or modifying existing files is causing the problem to come back. I have added a couple of new albums and the GROUPING is not working for those, but still OK for previous albums.
And the same goes for ROONALBUMTAG: modifying some existing files to add a new value to this TAG is causing the problem to occur again for those files but unchanged files are still OK.
So the fix seemed to be OK until I have started changing the existing multi-text TAGs by adding new values or modifying existing ones.
