Cannot fit through trapdoor

Screenshot attached. I have a ladder leading to a trapdoor to my roof. No matter how I wiggle, I cannot fit through the trapdoor. I have tried both from below and from above. I have also tried removing my armor and switching to the Steve skin. I am not using any add-ons.

In 1.2.0 I was able to go through with ease, with just a normal amount of wiggling. I can still do so if I remove the trapdoor. So it seems like either the trapdoor hitbox or the player model has gotten a tiny bit larger. I sometimes have trouble going through regular doors, too (not very often), so it may be the latter.

Edit: In case it matters, the trapdoor is attached to a bottom slab.

Edit 9/30/2017: This occurred in my main survival world which was first created more than a year ago. When 1.2.1.1 came out I saw it had a lot of bugs, so to hedge against corrupting my world I made a copy. I had been playing that copy for a while before I discovered the bug. But yesterday I made a new copy of the pre-1.2.1.1 world and tried going through the trapdoor. I HAD NO PROBLEM. So whatever caused the bug in the first copy appears to have been introduced DURING 1.2.1.1 GAMEPLAY, which means it's probably not reproducible.