It seems likely that Roon has not been set to prefer your file tag for Album Artist. You can change that on an album by album basis as described here. This KB article has more information about editing.
You can also now alter your import settings to globally prefer your file tags.
This KB article has more advanced information about treatment of file tags in Roon.
In the situation you describe for Aphex Twin, I’ve struck the same issue with Tribute or Remix albums. I like the Artist attribution by Track and also Album Artist as “Various Artists”. But I would like to see such albums somewhere on the relevant Artist page and not have to delve into Composer. My solution is to add a Featured Artist credit for the subject Artist:
This results in the tribute or remix album being included under “Appearances” in the Artist entry.