This is primarily an autism focused question, although allistics please do chime in if you have something interesting to say!
I’m not referring to stuff like Therians or “not feeling human”, although those are deeply fascinating topics. No I’m strictly referring to finding the task of imagining the human form (in say “daydream stories”) to be unpleasant/uncomfortable.
When I was real young I was REALLY uncomfortable imagining the human form. No problems with animals and weird creatures thought up by toddler me, those didn’t make me uncomfortable!
I brought this up with my IRL friend who’s also autistic. He also experienced something similar!
Anyone else also do this? I kinda want to just… Talk about it. It’s interesting.

I’m afraid I don’t really understand what you mean here. Are you saying having a picture of a human form in your mind makes you uncomfortable? Any bipedal humanoid shape or maybe the naked human body specifically? Or are you just made uncomfortable by the idea of a hairless vertical entity with jointed tentacles?
Realistic depictions of the human form. Clothed and unclothed. It no longer bothers me, but it really did as a kid.
I prefer visualizing more cartoonish/abstract depictions of the human form (Red’s style on “Overly Sarcastic Productions” seems to be my subconscious’ favorite go to) as that never triggers anything. And I was always fine “playing with” inanimate objects and animals inside my head.
Weirdly imagining “realistic humans” in the context of reading a book was never uncomfortable for me. Only if I went out of my way and did it myself. Very weird, but also very interesting!