iTunes stopped writing id3 tags into MP3 files

How can I be sure iTunes resumes writing id3 tags into each MP3 in my library?

I recently realized iTunes stopped metadata into each song's id3 tags. The files are standard MP3 and AAC format which have never had problems before and are perfectly editable in other programs. In iTunes, I'm no longer able to add album artwork, it disappears as I close a song/album Get Info window. When I try to update album or artist tags, the Save File dialog doesn't appear. Essentially MP3 files behave like WAV files.

Consequently, when I copy songs to my iPhone, the artwork is missing or compilation albums won't stay together.

The albums and songs look OK in iTunes browser, but the metadata is only stored in iTunes' database - not each file's id3 tags. I can copy the songs to another computer or application (even another copy of iTunes), correct the tags then re-import the songs, but that's not ideal. My main iTunes library has grown over the years, amassing lots of carefully curated metadata. I'm bothered that tags for some of the last few albums I've imported/edited are not actually there. It's time consuming to figure which ones are incomplete and re-enter the information. it seems like the problem may have started around the time of the last iTunes update but I can't be sure.

iTunes version 12.8.0.15.0

Library size 565.38 GB on an external drive

2012 Mac mini running macOS 10.13.6

Permission are correct. The external drive hosting the library is functioning properly. All software is up to date. I've tried rebuilding my iTunes library from XML backup which seemed to correct the issue briefly but iTunes again resorted to not updating tags. I've even tried reinstalling iTunes. The problem only appears to affect this one Mac iTunes installation.