It’s not totally uncommon with reproductive organ abnormalities in rabbits. A bun can have no uterus or an under-formed uterus. Did the vet mention if he saw any ovaries? It could be possible that your bun has ovaries but no uterus. In which case she cant get pregnant or get uterine cancer (of course), but she could still become hormonal because sex hormones are produced by the ovaries.
Dont feel bad because you took her to be spayed. Vets don’t do routine ultrasounds of young does to check if they really have the organs they’re supposed to have. Neither you nor the vet could not have known.
She seems to be pee marking in the space that isn’t her own. That would pretty much rule out a UTI because a UTI would cause her to pee in random places in her own room as well. Pee marking is hormonal to a great extent, but it can occur with neutered/spayed buns as well, especially if the household has other rabbits or other pets.
Sitting in her own pee is not pee marking or hormonal though. It’s generally associated with pain of some sort that makes the bun unwilling to move. It sometimes happens when a bun has a tummy ache (gas). It can also happen if a rabbit is very fat or very old (age-related joint problems making it hard to move around).