The bug

For each model file, ambient occlusion can be either enabled or disabled. When models are incorporated into multiparts via blockstates, the ambient occlusion value of the first stated model is used, rather than each individual constituent model either using or not using ambient occlusion as per each model file's settings.

How to reproduce

A resource pack is attached to this ticket. Crimson fungus is given a model that uses ambient occlusion, and warped fungus a model that does not. The dead bush and cobweb use multipart blockstates files which combine both: for dead bushes, the crimson fungus is first, whereas for cobwebs, the warped fungus is first.

1. Download and apply this resource pack
2. Place crimson fungus, warped fungus, cobwebs and dead bushes

Expected results

Dead bushes and cobwebs would look identical: one half would obey ambient occlusion and the other would not.

Actual results

The entire dead bush model obeys ambient occlusion and the entire cobweb model ignores it.

How to fix

Make ambient occlusion settings apply per constituent model instead of only depending on the first for the entire compound model.

Further notes

It may also be advisable to make ambient occlusion a per-element setting, rather than a per-model setting.