After the mind-fuck of Tim’s sixteenth birthday, Bruce criticized Tim for believing in time travel. Fucking time travel, when one of his best friends is Bart. Who is from the 31st century.
Or - is he? Does Bruce know something he doesn’t? Is Bart lying to him? Is he hallucinating? Falling for some supervillains trick? If any of the Bats could turn, so could anyone else he knows. Maybe they already have. Maybe they were never on their side in the first place. How can Tim possibly know?
(Congratulations, Bruce. You just managed to break your sidekick. Good job, we’re all impressed.)