When they have been using the litter box properly and then they start NOT usng it, you need to have the vet check them out for a URI = urinary tract infection; and for crystals. There will not be any other signs, so you may think they have no health issues, when in fact peeing out side the box IS the health symptom.

Once the physical is eliminated, then you work on behavior matters. I know you said no big moves or changes, but . . . did you move the litter box? Change the litter you are using? Start cleaning the box at a different time? Did you get a new litter box with a hood and the prior box had no hood? WE humans do not consider these big changes, but these are HUGE changes to cats.