riotbzim almost convinced its because of this
The projectile hitboxes are collision hulls. These are the same as the bounding boxes that are used to calculate collision between players (and I'm assuming players and the world).
I'm pretty sure wallbugging only works on walls that are diagonal relative to the reference frame of the map because the corner of your collision hull is getting stuck in the wall (collision hulls don't rotate with players, they're fixed relative to map coordinates).
A lot of weird stuff happens on the diagonals of collision hulls, pretty sure they're responsible for a lot of facestabs and people meleeing you from 5 miles away because you still count as colliding while being 40% further away from the player model.