In the WN, she actually first encounters the treant before this point. While she was burning from the dragon's magic flame. It just suddenly reveals itself to have been one of the other trees in the clearing the entire time, and then promptly abandons her. That's why she initially gets the idea to eat it, as payback (and she definitely would have if Kiri hadn't interrupted).
The manga isn't a 1-1 adaption, so it's understandable, but it can still feel a little jarring when they have to rewrite events in a way that accounts for leaving some stuff out.
Anyway, I haven't seen the raws, but I just realised what's probably going to happen in the next few chapters. That'll certainly be.... Interesting.