Yes! It seems to be the Work field!
Take a bow, tt2. I am very grateful for all the help you give our community!
I am currently testing with 18 'bad' or problem files and one good one - which doesn't always sync, unlike the bad ones.
Indeed, the Work column was empty only in the single good file and non-empty in all bad ones.
So, I deleted Work field data from one of the bad ones and now it is good!
Also, the Work field does appear to contain data that was originally in the Grouping field.
I have usually set edited the Grouping field on imported mp3 files, to contain the date of a recording, if known, or, if that info was unavailable, the date of release. However, sometimes, I have set the Grouping field using tag editors other than iTunes, such as mp3tag_s.exe which shows this ID3v2.3 field as TIT1 (Content group description) as per the ID3v2.3 spec., as I recall.
Furthermore, looking at my entire collection, I am tempted to simply erase all of the Work field data or to selectively copy and pasting 'Work' field data back into Grouping in iTunes, but that will be a lot of work. I'd like to understand a few things to prevent this from happening in the future.
Previously, I had deleted all ID3 tag data using mp3tag_s.exe. Then, since this didn't fix the problem, I concluded that it had nothing to do with ID3 tag data, which is why I resotred to re-encoding from mp3 to wav and back to mp3: not do delete tag data but to re-encode streams, which I assumed was causing the problem. However, it appears that maybe mp3tag never deleted all the ID3 tag data in fields like Work, that it couldn't recognize? I don't know.
How did my Grouping data get into the Work field?
Does 'Work' correspond to a documented ID3v2.4 or ID3v.2.3 field?
In the past, when I manually edited Grouping field song info using iTunes, I was able to verify that iTunes wrote that data to the ID3 TIT1 field so it was never an issue what software I used to edit this tag - it always displayed in iTunes.
However, I was just recently reading something about iTunes writing ID3v2.4 tags instead of ID3v2.3. Is that true with iTunes 12.10.10? Has the default ID3 type changed over time? Maybe that is part of the problem...
Just now, I added some Grouping data to a mp3 file in iTunes with an ID3v2.3 tag that previously had no value in Grouping (as seen in iTunes) or in TIT1 (as seen in mp3tag_s.exe, however, now after inspected it in mp3tag editor, instead of showing the data in TIT1 ID3v2.3 field, it displays the message:
> Unkown frame type (GRP1)
So I deleted the text in iTunes and re-inspected and the "Unkown frame type" is gone.
I am trying to tread carefully here since some day, hopefully, I will be able to port my metadata to a better platform so I'm leery of mixing TIT1 with "Unknown frame type (GRP1)"
Incidentally, I looked at the other link you posted but I have no file permissions problems.