Why does my housebroken dog poop inside immediately after going out?

It's not uncommon for temporary housetraining setbacks to happen, even in adult dogs. I would recommend going outside with the dog when he goes out to potty, and rewarding with praise and a tasty treat for pooping outside. Until this issue is sorted out, he will also need to be supervised at all times to prevent him from having the opportunity to poop in the spare bedroom - if you notice him starting to sniff the floor or circle, hurry him back outside, then praise and reward for pooping out there. You may need to use a baby gate or other physical barrier to keep him in the same room with you so that you can watch him.

The "guilty look" you are seeing is actually appeasement behavior - dogs do not understand the concept of guilt the same way we do. This dog has likely learned that he will be punished if there is poop in the house, but this is not the same thing as understanding the cause-and-effect relationship between the act of pooping and being punished. In fact, most dogs who have been punished for housetraining accidents will show this behavior even if you bring some poop from outside and place it on the floor - you can try this sometime if you have doubts! :)

I would stick to this plan until he has gone at least two weeks with no accidents at all before allowing him any unsupervised freedom in the house.