This always made me wonder. Jiren clearly lost to UI Goku, but he still won the "fight" because Goku depowered before he could finish him off. The manga is different, where Jiren doesn't actually lose to UI Goku, but I'm not going to go into that.
Jiren could have at any time during the tournament knocked Goku out, he just chose not to. Jiren allowed Goku to get pummeled, giving him more reason to fight harder, giving him more opportunity to power up, then just left him alone. The same thing happens when Goku goes imperfect UI and Jiren still holds back.
So is Jiren just retarded for whittling down his strength instead of going for the ring out? Or is it actually his pride (pride trooper) that made him want to push his opponent to the limit in order to show off his own superiority?