See user report here: https://discord.com/channels/602796788608401408/1454570451862032394

I have a reproduction using this world and this resource pack on dev.

Analysis: The exception at this line is being thrown because earlier at this check I mistakenly assumed there could be no quads where the onPlaneMap is 0110, but this can be the case for when bent quads are split in a very particular way like this: https://www.desmos.com/3d/vuvamwcupk

The solution is to simply account for this special case in splitting and generate one degenerate quad triangle for the quad triangle that's on the inside and split the triangle that's crossing through the split plane. This is equivalent to one of the regular "vertex being clipped off of a quad resulting in a triangle and a pentagon" mode.