If its out of warranty get some Electrical Cleaner (its nonconductive so don't just use any old cleaner) quick squirt down the gap and all will be well again, works on my X1 so no reason it won't on the U2.
I only have the X1 and its always left in Activities and stays there sometimes for weeks when on holiday so unless they have changed the way they work (I hope not) I would say you have a faulty one and as it’s the X1S you may still be in the return window, if so return it and get another.
This is quite common, if its still under warranty e-mail them with a video of the issue and they should replace it. If its out of warranty or you just wish to fix it get some Electrical Cleaner (its nonconductive so don't just use any old cleaner) quick squirt down the gap and all will be well again.
If its out of warranty I would get some contact cleaner and squirt a bit down the top "Back" button, it could be getting stuck, you will probably need some contact cleaner anyway when the wheel starts playing up.
If its in warranty I would contact them throw e-mail for a replacement.
Try doing it in the dark! I have a new and an old Panasonic remote both will operate my TV (so same codes) but the X1 will only learn from the old one in the light. I have another remote it won't learn from at all but if I teach an old learning remote I have the codes first then the X1 will learn them from that remote. My theory is that on my old remotes you can see the transmitter but the modern ones have dark plastic hiding them and the X1 hub also has dark plastic hiding the receiver.
That was the setting I was on about. Odd you had it set the same way I have it but the behaviour is different. CEC could be confusing things but then I would expect it to interfere with “Always on, unless the Off key is pressed” as well. But if it’s now working how you want it that’s what counts.
