For some reason on PC, quads seem to be triangulated in the opposite direction from PS1. This appears to be the reason why things such as Cage Skip, Upwarps, this ledge in Cossack Hub, etc work differently between PS1 and PC (discovered by limbus 2024-09-30)
(implement this info from OverSurge and Rartrin on how the triangulation is done on PC vs PSX)
Here is a 2D example image, showing the two ways a quad can be triangulated (source)
(add some in-game example images; there are some around here from limbus)