Pooping in the middle of the night

Hello. Neither of my dogs have been crated. Both were able to hold themselves through the night when they came home for the first time. Fast forward 2 months [and a few days after neutering], the baby [6 months] is pooping in the middle of the night, in the bedroom. We already go out for a late night walk/outing before we go to bed each night, which is almost always 11 p.m. I just started to chart the times he poops [something I thought I already knew]; and am considering changing their feeding times [ currently 6 a.m. and 4 p.m.] but wondering if anyone else had a similar experience...or knowledge of a connection with the surgery and behavior. Thanks for any advice you can give.