12 yo Shih Tzu randomly forgets how to come inside through doggie door

I posted this in dog advice a couple weeks ago and didn’t get any answers, so I thought I would try here. This is the original post:

I’ve had my Shih Tzu since he was a puppy. We moved into our current house about 7 years ago. The house came with a doggie door preinstalled on the back door. After a day or two of training, he has used it without issue for years. Now about a month ago, he randomly has sat outside barking for us to let him in sometimes. When it happens, it will typically happen a couple times the same night (he only does this at night by the way), and then he will go for days without it happening again. I thought maybe there wasn’t enough light out there, so we have tried turning on the patio light. But even with the light on sometimes, he still will just sit there barking at the door. My wife and I have both tried opening the flap and telling him to come inside. He just sits there confused and will whine for us to open the door. It’s just weird because it is random like I said, but when it happens, it happens frequently, and then it just stops and he’s fine for several days, and then it will randomly happen again.

And an update since I posted this:

Over the weekend, we were trying different things to see if we could open the doggie door from the inside and hold up a treat for him to come through. He will come up to the doggie door sniff at the treat, and then act scared to go through even to get the treat. Then we tried going outside to see if we could help lure him back through, and when he sees us outside, he will get excited, and run through his doggie door to come back inside and play. So clearly he knows how to use it still, he’s just not wanting to for some reason?