Since the 5.1 patch, I fail TCJ more often than before. It triggers the cooldown, but I never see the box appear on the ground. Either it's bugged, or it's become much more sensitive to movement, and I'm accidentally hitting a movement key after casting it.

How about:
Allow movement within the box, and fail the TCJ only if you move out of it. This would avoid accidental cancels, and might allow limited dodging of AOE's as well.