Held items only render if the entity geometry is exactly called "default"

If an entity is holding an item (and meets other strange and undocumented conditions), it is supposed to render that item in its hand. However, this only works if the entity is rendering a geometry that happens to be given the friendly name of "default" inside its client entity definition.

This makes it impossible to render items in the hands of entities that have multiple different geometry variants, since all the variants can't have the "default" name. I'm not aware of any documentation regarding special hardcoded properties of particular friendly client resource names.

How to reproduce

1. Download and open the attached world

2. Look at the two zombies. The one on the left is test:test1, and the one on the right is test:test2. They are completely identical, except that test1 uses default for its friendly names, and test2 uses anything.

Expected behavior
Both zombies should be holding a sword

Observed behavior
Sword is only visible in test1