If you close a fence gate / trapdoor or any other block you shouldn't be able to stand in you'll have the normal visuals in first person but in third person you'll see you'll stand up like normal. You'll also walk like normal and with normal speed but if you leave the block you're under its starts crouching again like normal if under a half slab or you'll stand up like normal. The visual bug wont be seen in multiplayer by others but you'll still be able to walk around normally while you should be walking around when crouching.