Well, let me ask you this. What would you have them do? If it's NOT a reproducible problem - and they tried to do so - and there were no error codes that showed anything (and on this, you can rest assured that the Subaru Quality Team IS trying to make sure that they are on top of everything with the Ascent) - then there's not much else they CAN do. It's not a lemon - the car had a single error and nothing else has happened. (I've HAD a lemon before - one that we made, even. An Isuzu Rodeo with a transmission that would NOT stay in 4WD. It slipped from 4WD into neutral while on a hill in the winter with a semi behind me. The dealership tried to fix it, the factory took it back in and put a new transmission in it, it still would not work right. Lemon Law and bought a new Outback - that the paint wouldn't stick to, so the factory had to repaint it.)Our problem is what we were told by both the dealer and corporate - unless the dealer can reproduce the issue (video or not) there is nothing they can do. Unfortunately Subaru has not taken very good care of us as owners of a $45k car.
Literally the only thing anyone CAN do is wait to see if it happens again. It may happen tomorrow, it may never happen again. One of the things when the engine throws a fault code is that most or all of the red lights come on - even if there is nothing actually wrong with that system. The transmission fluid isn't going to overheat within a few seconds, and since the stuff does change color if it HAS overheated, all the tech has to do is look at it (and smell it) to know it's fine. Could the CVT chain have slipped for some reason? Could the computer that controls the pulleys messed up somehow? Who knows? What would you have them do? Replace the transmission? Simply because it made a non-reproducible error? There's NO manufacturer going to do that.
And yes, I work for a dealer as well, I've been very active on this forum since well before the cars ever got into customers hands, and I used to work at the factory in Indiana building these things. If there is a problem, there is NO other manufacturer that will step up and do the right thing like Subaru does. They just have to have something to work with - and right now they have nothing.