As of 1.21.4 update there is a different way models are rendered. This change negatively affects item model customisation.
In the previous versions it was possible to change model for item for example by simply specifying that you want to derive from a different item model:

{
    "parent": "item/stick"
}

If you try this approach on an block-entity like for example bed (white_bed.json), you will see that it doesn't change anything.
The bed now renders the block model instead of the explicitly specified item model.
I tried this even using the new method for creating item models and nothing works. The tested resource packs are provided (diamond sword and white bed should look the same).