I don't think we can assume this is a case of Jack being problematic (although I'd never deny he has been in the past, especially in his youth).Do we have an English player that isn't in someway problematic off the pitch? They're all bellends away from football.. young lads excluded..for now
It might simply be that he's had a niggly injury that he's just about recovered from now and the club and country's medical staff are in agreement that he should be fit enough to feature in the international break.
The only reason anyone's speculated it's anything else is that Pep hasn't given any details of an injury. But maybe he's just respecting his privacy with regards to his medical record.