"My uncle is an artist" is a classification - you are classifying your uncle as an artist.
"Paul is my artist" is an identification. The word order for a classifactorial clause (x is a Y) is not the same as for an identification clause (x is they Y).
The pronoun follows the copula in identification sentences. See: IDENTIFICATION at this link: https://forum.duolingo.com/comment/7068975/Eureka-Moments-with-the-Irish-Copula-A-Crib-Sheet Also, it's identification because "Paul" and "my artist" refer to the same person. Now you'll get those!