This is an invalid issue, because each UI screen is parsed manually, therefore the game doesn't support custom screens. Perhaps this is a feature request.
To replicate the newline character, use "\n". ;D
The issue is actually the exact opposite; The BLOCKS appear lighter than their slab/stair counterparts.
Still occurs in the 1.20.30 betas.
This was added in the better together update, and will be present in every future version the inventory_screen.json UI file is not changed.
Affects 1.20.30