The bug

If a model file uses the "parent" parameter to reference another model, and the empty string "" is used, the observed results are inconsistent.

How to reproduce

A resource pack that makes oak leaves point to "" as the parent model is attached.

1. Download and apply the attached resource pack
2. Obtain/look at oak leaves

3. /give @s minecraft:birch_leaves[minecraft:item_model=""]

Expected results

Both oak leaves and the given item for birch leaves reference the empty string. Therefore, either both would appear empty, or both would use the missing model.

Actual results

The oak leaves appear empty, but the birch leaves use the missing model.