The bug

Back in 1.14 and before, in first person the item's on-model shadows changed accordingly to light (blocks that glow, the moon, the sun), but now the shadows don't change, not even with a really near light source.

The only item that I found that works as it should is the trident, the shadows change accordingly to the enviroment.

Update: The bug has been partially fixed in 19w45, but the lighting still behaves a little weirdly.

To reproduce

1. Be in first person mode
2. Pick any item and then hold it
3. Pick any source of light (the sun, the moon, a torch, etc)
4. Move your point of view in relation to the source of light.

Do this in 1.14.4 (or any previous version), and then in 19w42a