Probably just ask about it. They could be asexual. Years ago I was with someone who was, and we had sex, a lot of dirty fun, she was totally into it, but she didn’t cum and told me she doesn’t need to. I found out after that she loves having sex just to feel close and connect, not for sexual gratification, she didn’t masturbate either.
https://en.m.wikipedia.org/wiki/Gray_asexuality
Without knowing your relationship history, maybe your partner is somewhere in this space but doesn’t openly talk about it?
Probably they invented pavlova