what does the tag inspector say about these?
Thanks psychoadept. I can see why this is happening now.
The files are in different formats, mp3 and ogg, but I couldn't work out why some tags with multiple entries (m.brainz artist id) worked and others didn't.
Because technicians is using TIPL, a predefined ID3 tag, it gets saved as a single string in mp3 files ("Producer:Ronald Hagen;Producer:Pascal Minnard;Producer:Scott Mac;") as dictated by the ID3 standard. In ogg files the tags are separated into 3 entries buy the "; " null separator and thus don't match the mp3 tags and are correctly seen as mixed tags.
"m.brainz artist id" is a custom TXXX/ tag so it's saved as separate entries in both mp3 and ogg files and seen by MB as being the same across multiple files.
Thanks again for pointing me in the right direction.