Reproduction steps:

1. Enable this tagger script:

   $delete(ARTISTS)
   $set(artists,%artists%) 

2. Click the tagger button on https://musicbrainz.org/recording/59b53425-6aa7-45ef-95fa-22819ce08ed2?tport=8000
3. Drag an empty MP3 into the standalone recording. Picard will show that it will add the field "Artists".
4. Save the file and refresh. Picard will show no changes. However, if you look at the actual file, the "Artists" field was not actually written to TXXX:ARTISTS.
5. Delete the standalone recording release from Picard and redo step #3. Steps #3-#5 here make an infinite loop.