Selective Potty-Training?

My 5 month old mini aussie was getting so good at going to the bathroom outside. We have a 24/7 open doggy door, and he goes outside, goes potty and comes back in.

But randomly (like today) he'll just pee inside or poop (and eat it, but im aware that's rather normal for puppies and they'll grow out of it). We got him to free roam the house at night, and had zero accidents, and would go outside when he needed to.

Any idea why he goes to the bathroom inside AND outside randomly? Any tips on how to encourage him to ONLY go outside? (Our other aussies were a bit older and pre-trained; we never had an issue till now)