Since iTunes release 12.5.4.42, it read & writes iTunes Grouping to a new id3 frame "GRP1", while the TIT1 frame is dedicated to the Classical "Work" field. If and when Mp3tag adds support for the GRP1 frame then we will have direct access to iTunes Grouping again.
There is a yet unanswered question on this forum on whether there are plans to support the GRP1 frame in Mp3tag. It seems logical to eventually support it, although Apple did breach the id3 standard by making up their own frame instead of using a TXXX frame, but the truth is no one really follows the id3 standard to the letter anyway, common convention has been more the rule rather than the rules themselves.
Support for new iTunes Grouping id3 frame "GRP1"?
Without actual support for the GRP1 iTunes Grouping frame, one workaround is to use a hex editor to batch edit all mp3 files and change "TIT1" to "GRP1", then reimport into iTunes. The first time you do this you will have to clear out WorK in iTunes Get Info to clear the Work stuff out of the iTunes library, but it should stay out after that. Also need to be using a copy of your files exposed to iTunes, keep your originals that Mp3tag references elsewhere. A hex editor can corrupt your files if you don't know what you're doing, proceed at your own risk. That said, I tried it and it worked for me.