Certain blocks use their 2D item texture instead of a 3D block texture when held in hand

Blocks like a cauldron and a brewing stand use a 2D item texture instead of a 3D block texture, unlike most blocks. If you take an oak wood plank (for example) and compare it to the texture of a cauldron while it's in your hand you can probably tell the difference, the cauldron does not have a 3D block model.