Its probably because he had his horns cut off twice. Probably messed with its development since the horn affected his brain.
it gets explained in the novel.
Spoilers:
the sac already grew and was expelled when Hugo was Andrea ╮(￣▽￣)╭
I think Plin ate it
Oh that makes sense
Ooo thanks for explaining
Yeah it was explained that its not just love but also grows as a result of heat/rut.
Essentially young Andrea was lusting over Plin for a while and made Hugo jealous over his younger self. Also made plin realize Andrea wasn't the innocent boy he remembered.
What's worse is, the author didn't explain or like show the sac? Idk thing not growing even when he loved mc