I've noticed that if I put a trapdoor adjacent to a block, it becomes transparent and using another trapdoor with "window" I can see through walls. The issue became more obvious when I loaded 3D models for trapdoors (I used models from vanillaweaks.net). Then i could see through a wall straight out the trapdoor with a gap.

This issue has been raised already and is being investigated under #225

Btw, mangrove trapdoor looks kinda cool with that