I think the link below refers to the problem you describe:
https://forum.audacityteam.org/t/mp3-metadata-only-saved-to-first-track-when-using-export-multiple-based-on-track-names/119179
The issue has been around for over a year, I think. You need to find an external metadata editor for Mac (similar in function to MP3Tag on Windows). ChatGPT suggests something called Kid3.
Mark B