Jack had taken one whiff of this grove and bolted from it, earlier today. He knew the smell of a predator when he caught it. Here and now, the wolf is just as wary. He doesn't feel fear, precisely, but he can feel apprehension, and when tentacles exchange for vines, he attempts to tug free, snapping his teeth at anything in range. He doesn't want to be prey! That isn't what he's meant for!

If John is slow to pull his tentacles away, he might get bit again, but it'll give the vines enough time to hold him fast, especially with the grove sapping his supernatural strength.