When using polys: "tri_mesh" or having an array size of 3 it will continue to render as a quad rather than a triangle using the next vertex possible and if only a single triangle is present it won't render at all.

Below I attached a intended result in block bench an in game image and the geo used to render it in. To make it more see able I put the further point first in the array making it a square

Edit: Add The full pack from with a texture and geometry change
https://mcpedl.com/poly-mesh-models-in-minecraft-bedrock-example-addon/