I'm having some issues using Tag "Content Group" (TIT1) (called Work in iTunes).
I can update them one at a time using either the tag panel, or the relevant column in the main window, but I can't multi-edit like I can with all the other tags (that is multi-click on files, then change the value in the tag panel).
It is not quite plausible what you describe.
Either the files can be modified - then it does not matter if it is just one file or several - or they can't.
So please tell us what you do and when you see that apparently it has not worked the way that you expected.
(Please note that you have to explicitly save modified data when you use the tag panel but you probably know that already, if you can edit other fields like that).
Oh yes: what kind of files are we talking about? FLAC, mp3, mp4, others?
Check whether you have APE tags in your files with the filter
%_tag% HAS ape
If the file list is not empty and even shows the files where you try to add the content group field then check in File>Options>Tag>Mpeg
what kind of tags you read, write and delete.
If you read APE but you do not write them, then you see the unmodified contents of the APE tags. And as APE tags have priority over V2.3 tags in MP3tag, you see their contents and not that of the V2.3 tags.
You see me at almost at my wits end. I just added a field in the tag panel for CONTENTGROUP, selected several files, added some text and saved the data - and it was displayed in MP3tag.
What happens if you do the multi-edit in the extended tags dialogue? Does that also fail?
Could it be that you have 2 fields for CONTENTGROUP in the tag panel?
And if nothing like that works then the only thing that I can recommend with some shrugging of my shoulders is to check the files for integrity with one of the utilities linked here: