Toilet Trained Puppy Started Pooping Inside?

We have an 11 month old English Staffordshire terrier. On a regular basis we dog sit our friend's Staffy (1 year old) for a couple of days once a month.

Recently our puppy has pooped inside the house when the other dog is staying over. He has done this twice most recently and at seperate times.
The first time he pooped where the other puppy had previously been sleeping, but at the time of execution the other puppy was on our bed sleeping next to us. They both end up sleeping on our bed through the night which is normal.

The second time was inside on the mat to where the main exit to the outside door is. This particular time he proceeded to exit the house through the recently installed dog door and put himself in his kennel. There was a little trail of poop leading towards the dog door.

I'm not too sure what is going on, is our puppy jealous of the other puppy? Your advice would be greatly appreciated :)