Over the past few months, probably since the release of Mojave and iTunes 12.9, I've occasionally received queries from Correspondents concerning a problem with changing the tags of MP3 tracks. The changes wouldn't be written to the MP3 files' metadata or would revert back to what they had been before the change. It affects MP3s only, not M4As.
I was not seeing this myself nor was I able to replicate it, but, as I say, I was asked if I knew about it a few times.
This post at Apple Support Communities appears to have discovered a factor involved: AirPlaying to AppleTV. When AirPlay to AppleTV is turned off tags would be written correctly to the associated MP3 files.
I'm wondering if this could be related to another MP3 issue I have heard about recently. And this is weird. Whenever a script of mine is used that applies artwork data to an MP3, the file is mangled in such a way that the last several seconds of audio is copied and added to the end of the audio file (I said it was weird). I could not replicate this either.