Custom created or 3D modeled blocks glitch out from most perspectives. As of versions 1.16.100 and 1.16.200 this was not an issue, though as of 1.16.210 they will appear hollow, transparent, and flat out broken from many angles, and completely normal from others. They sometimes flicker between looking normal and looking bizarre if I move around them.

(Shown below: regular opaque custom block as it should look, glitched regular opaque block, and a 3D modeled block with the same problem)