Overwritten client entity files for tridents and shields no longer display custom models in hand

Partner Rank:

3

Partner Team Reporting:

Everbloom Games

Verification builds:

1.16.210.58

Summary:

Overwritten client entity files for minecraft:thrown_trident and minecraft:shield no longer display custom models when their respective items are held in the hand

Impact:

We use this trick in Ninja Mash-up and Ninja Parkour to display a custom grappling hook model in hand when holding a trident; an upcoming map that reuses the grappling hook; and another finished map waiting to release, which uses both items to show arbitrary models on an entity's hand.
Now, in all these applications you just see a trident and shield instead of the intended models.
Additionally, trying to get around this by overwriting the geometry instead of the entity results in the model not having the same offset as before.
 
Repro Steps:

1. Open Ninja Mash-up
2. Hold the grappling hook in hand.

Expected:

It should display a grappling hook model and texture.

Observed:

It displays the default trident geometry.

Screenshots/Videos attached:

Yes

Regression Builds:

Last worked in 1.16.210.57

Notes:

At the very least I'd like to see this change versioned so content made before R16U4 isn't affected.