Items given an item_model component that points to a semitransparent texture will render as opaque in the world if the original item is a block with an opaque texture.

To reproduce, use the following command:

/give @s minecraft:dirt[minecraft:item_model="minecraft:white_stained_glass"]

The item will render as normal in your inventory, but when held in your hand or when dropped into the world it will render fully opaque.