my cat too has this problem, she sits on her poop. i thought i was accidental but after a few observations it has become a behavior of hers to sit on the poop in the litter box. so i keep watch when she starts to go to the box and do hr business, i immediately go to her and hold her tail up a bit so she can't sit on the poop. this has become our routine for months now but she has never learned not to sit on the poop til now. i didn't consider trimming her behind since it doesn't solve the problem. a possible solution though is to potty train her and replace the litter sand with just water just like in a toilet. i have modified her litter box to have a access hole big enough that she won't sit in and have the old used litter sand in. so far she pees on the access hole platform often missed the hole. and often she pees outside somewhere else. so i still return to her old litter box. my vet says its a behavioral problem and that i have to patiently train her, to this day i have no luck yet. i'd be happy to hear of your observations and working solutions.