The sides of trapdoors are invisible from iside of the trapdoors
i made a video
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This is intentional, I recommend to you to post this on the feedback website
It already has a report saying that Mojang considers it not something they are going to change

I also recommend https://vanillatweaks.net/ (Created by Xisumaviod) because they have textures that "fix" this "problem" in the 3D Section in resource packs.
Duplicate of MC-2333