What are the details of the training and/or management of this little pug? I'd be interested to know in detail what the training process is like. Also, what does this little dog do during the day? How much mental and pysical stimulation, walks etc is he getting?
I disagree that anxiety or stress has nothing to do with it. If the dog is affected by moving from place to place, changing care takers, upsetting his routine, changing diet...any number of stress factors can definitely cause regressions in potty training and elimination directly due to stress. I've seen many dogs forget what the heck they're suppose to do when changing addresses. They don't generalize very well and potty training might be down pat in one home, but in another home, it's completely foreign to them. It is true that many dogs who are traumatized don't have bathroom behavior regressions. But this is one of several ways anxiety is expressed. Some do and some don't.
My own very well house broken Chi's (at home) get stressed when I leave them at my ex's for him to care for. For 3 days, they're fine. After that, if I don't return, they begin having an accident here and an accident there. He says he takes them out, gives them a chance to go and frequently.... and they come back in and immediately have a boo boo. It's always after about the 3rd day. They're stressed when I am gone too long. And it appears that it's expressing itself in this manner.
There must not be any punishment associated with potty training, whether caught in the act or not. There are all kinds of detrimental side effects caused by the use of punishment which will interfer with not only the relationship, but with potty training itself. If the dog has an accident, it is the owner's lack of diligence in supervision, not the dog's fault.
Constant supervision must be in place, even to the point of tethering to your partner's waiste. A crate used when the pug can not be supervised and I mean if it's just for a phone call where he will be distracted for 1 or 2 minutes (pug goes in crate) or if he himself must make a quick trip to the bathroom. Dog either goes with him or goes in crate.
The dog needs to be taken out more often than it would seem needed, like every hour and not only praised when he goes, but given a high value treat the second he finishes...in addition, 10 minutes or so after eating, drinking, rough housing, napping, etc.
"Accidents" must be prevented and soiled/peed spots cleaned well with an enzymatic cleaner. If the dog smells a spot where he went before, to him, that IS the toilet and proper place to go.
In other words, the dog must be treated as though he were a new puppy just beginning to learn the ropes. He must be started from scratch. He must be set up to succeed and thus, have many more chances for reinforcement. (high value reward with each outside elimination) He must be shown what place works best. And prevented from guessing which room of the house might be the right place. This room, that corner, how 'bout over here....NO. That's too confusing. One place works. And that is outside. Yummy treats associated many times with going outside will communicate to the dog that outside is THE place to go. If absolute diligence is used, accidents inside prevented in the first place, this behavior should turn right around in a matter of a couple weeks.