I have something similar, and have also not been able to put my finger on it. I've had it for a while though (some months at least).
One common occurrence is stopping for Supercharge. Both wife and I get out, go and get a cuppa, and when we get back in the car the Climate is up on 26C or similar (having been left at say 21-22C). I have definitely not set Climate using APP in that situation. However, quite possible that Wife got to car first and her Fob change Profile from Me to Her (but she wouldn't have driven the car at 26C either, but it MIGHT have been set to her Profile when APP turned climate on,a nd since then car has been on my profile for the actual journey)
Also notice it when we are setting off. Maybe we already used APP to pre-heat the car. Get into car, adjust temperature (e.g. down to 21-22C) and then it jumps back up again. Not sure of circumstances but could definitely be that I get back out to do something - e.g. open trunk to rearrange bags or put something else in - and then when i get back into the drivers seat temperature is back up at 26C
I have wondered if it could be that setting Climate from APP is the cause ... i.e. that somehow causes the climate to be FORCED to a setting, again, a bit later on in the journey. I don't think it resets, by itself, whilst driving, only when Parked, but I wouldn't swear to it