Under Pressure was a Queen/Bowie collaboration. Spotify appears to tie it to Queen's catalog instead of Bowie's, probably because it was never on a Bowie album - it was released as a single in 1981, then Queen put it on Hot Space the following year.
I don't think the fact it wasn't on an original Bowie studio album (it's on a 'Best of Bowie' album by the way) should prevent it from being attached to Bowie's catalogue. And des that happen in any other case of a collaboration on Spotify?
Well, I'm glad you went down that rabbit hole as that is probably it - well thought out! It's a shame that Spotify don't have a way of integrating click numbers for the very same song/recording... Thanks