This bug exists exists since 2012 (https://tickets.metabrainz.org/browse/PICARD-376). Unfortunately, it has not been assigned yet. Hope that someone can look into it.
What happens is:
Artists (note the "s") shows up in Picard (artists = xxx; yyy). As described in the 2012 bug report, this tag is proprietary to Picard. When checking with DBPoweramp or others, it shows just as (artists: xxx). The second artist i dropped from the artists file. This issue only exists for MP4 files. Things work flawless for MP3 files.
Albumartists (note the "s") is not added at all though I have included $setmulti(albumartists,%_albumartists%) as script. This issue only exists for MP4 files. Things work flawless for MP3 files.