It doesn't always happen that way so it's not surprising you didn't notice it before.
Most of the time, your opponent can't back completely into the corner until you get within 1/2 screen distance from the corner. In those cases, they can be backed up into the corner as far as they can be and the TP uppercut will still hit. You'll notice the screen shifts towards the corner a little bit after the tp uppercut starts as you cross the boundary and allow the camera to move all the way to the corner.