I can tell you how to prevent it.
(Didn’t I explain it to you in a different thread? Might have been someone else though… 
)
‘Prefer local metadata’ is activated in the properties of the music library
Plex is expecting all music to be in a folder hierarchy:
Music → Album Artist → Album → Tracks
This applies to sampler/various artists albums as well!
So, if you have albums with differing ‘Album Artist’ metatags stored inside the same ‘Album Artist’ folder, then it is mostly determined by accident, which of these differing album artists will be used for all albums stored inside this folder.
In other words:
If you have e.g.
/music/Various Artists/Sampler 1
and maybe
/music/Various Artists/Sampler 2
you absolutely have to make sure that all files inside both Sampler 1 and Sampler 2 are tagged with the same ‘Album Artist’ name.
