As discussed with Grondag on Discord, here's an issue for it.

The symptom of this is that a QuadRotator that flips a model upside down will cause the AO-value for the top face to be computed from the down-face, leading essentially to an unlit top.

Example transformer: https://github.com/AppliedEnergistics/Applied-Energistics-2/blob/fabric/master/src/main/java/appeng/client/render/cablebus/QuadRotator.java#L77

Current workaround is to add this:

MutableQuadViewImpl quadImpl = (MutableQuadViewImpl) quad;
quadImpl.lightFace(rotation.rotate(quad.lightFace()));