So I finally understand why I have been getting unexpected results with both virtual tags and theater modes (what has been addressed with the latest patch) when using 'Composer'.
MusicBee is preferring a possible present 'Display Composer' tag over the actual 'Composer' tag.
It is not only doing that for displaying purposes at locations where that might make sense, but it is also doing that
in the Tag Editor panel, and for virtual tags.
Hm, I now realise I should always check the ... button at the right of the composer field to see what is shown there. Same as with 'Artist'. *
In my opinion that's incorrect and unexpected behaviour.
1. Can it be changed so that the actual 'Composer' tag is always used when a virtual tag uses <Composer>, and does not switch to using Display Composer if that happens to be populated too?
(perhaps <Display Composer> could additionally be made available for use in virtual tags so to have that option available too? But then again that may not be needed when 3.6 gets <Composers> )
2. Are there occasions where MusicBee will auto-populate the Display Composer tag by itself? (similar to Display Artist)
For now I am deleting all existing Display Composer tags from my library, which solves the current issues, but I am concerned MusicBee might populate them again at some point without me knowing.
* edit
3. A
+ sign should probably be added to Composer in the Tag Editor to indicate it when Composer has multiple fields.
Similar to that happening for Artist: