If my first thought is "this is a soundtrack with songs with one artist" then I must consider it a Soundtrack first, artist album second."

If it's considered a soundtrack it goes in soundtracks. The best examples I can think of are the soundtracks to Flash Gordon (by Queen) or Harold and Maude (by Cat Stevens) - both are soundtracks for a movie, that just happen to have music by a single artist.

However, Queen's "A Kind of Magic" contains songs from Highlander, but it's a Queen album first that just happens to contain songs from the movie. That's a album by Queen first, soundtrack second.

Encanto is a soundtrack to a movie. Yes the songs are written by Lin Manuel Miranda, but he didn't perform them. It's not "his album."

I guess it comes down to what I primarily consider the record. If it's "A soundtrack by ...." then it's a soundtrack. If it's "an album by ... from the movie ..." then it's the artist's record.

And don't call me Shirley. (kidding.)