Here is my code:

https://github.com/Technici4n/fabric-rendering-issue/blob/master/src/main/java/net/fabricmc/example/FourSidedFurnaceModel.java

Result:

Expected result:

The square method was meant (iirc) to use left-right coordinates from the perspective of the viewer, vs world axis coordinate. Doing it the other way is fairly straightforward using vertex coordinates directly.

Ah, okay. If it was intended to work the other way then this is indeed a bug :p

Your expected result expect the X axis to be flipped for each negative-axis facing. This should not be the case imo since it would mean that making a box with quads is more difficult since you can no longer use the same coordinates for the two opposite sides.