Unless you're incredibly perceptive and eagle-eyed, there's a solid chance you missed the moment Carl was bitten. It does not happen in the midseason finale, aka episode eight of season eight, aka the episode in which we learn about Carl's impending death. Instead, it happens in episode six, when Carl and relative newcomer Siddiq are ambushed by a group of Walkers as they make their way back to Alexandria.
It's a moment that's easy to miss. In this image, Carl is in the same clothes as that very scene, meaning this in-the-mirror moment likely happens just after Carl's fatal bite. Based on this, we can assume some, if not most, of the season premiere will happen in flashback; we're witnessing the precious few hours between the bite and Carl's death.