When remodeling items and utilizing Item Display Entities to display them in the game world, the model has backface culling enabled. This causes half of the model to not render when trying to adhere to the general Minecraft artstyle and model complexity.

Attached is the model as it was designed in Blockbench, as well as how it is rendered in-game.

Using single elements to create rectangular volumes is a fundamental basic of the artstyle, and being unable to replicate that seems to be a bug.