To me, it sounds as if Mowgli is trying to telll you something. Fister peed in our bed once - and right in front of us. He even looked us straight in the eye while doing it!

I'd get a urine sample to the vet to be checked for crystals.

Since you have several litterboxes with different litter, I don't think that is the problem, but you never know.

Best of luck finding out what's wrong.