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: