Unknown AlbumArtist showing in tag library for flac files with performer / composer

[new_bloke]

Thanks for the responses. I must say that the artist array seems to be focusing on a rather niche requirement - namely classical recordings. I'd prefer if it were either optional or configurable. Perhaps even the option to only apply this behaviour to specific genres. For now, I'll just update the affected files with a specific album artist tag.
At least I am reassured it's not me doing something wrong with the configuration...

There will be plenty of folk along in a minute to say classical is far from niche! 
An option to ignore the array and just use Artist could be n idea, although the number of Artist and AlbumArtist options is already high (I've my own track record of adding to the clutter in  these options)

I'm absolutely not suggesting that classical music is niche. I'm saying that requirement to munge artist / performer / conductor details into album artist is a niche one and that only really means anything for classical recordings, if then.
My opinion is that it's reasonable to infer Album artist from artist, if it's not present explicitly - that handles compilation / various artists recordings well. Adding in every single performer, is less appropriate in many cases, and doing that by default and labelling the result "unknown albumartist" isn't terribly useful - at least to me.
However, I can see that explicitly setting a value for album artist will make my issue go away, so that's what I'll have to do