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As the images show below, place two normal blocks in a row on the ground, place trapdoors at the bottom of each block and raise one trapdoor to it's 'up' position. Now jump on top of the block and sneak to the edge of the raised trapdoor- as expected you do not fall off the edge. However, when you sneak to the edge of the block which is connected to the lowered trapdoor, you fall down and if you continue sneaking to the edge of the trapdoor you still 'fall' off the edge.
Another concern which might be of interest- place a trapdoor at the top of the block and sneak to the edge of that trapdoor and you don't fall off. This therefore shows that sneaking and falling off the edge onto a lowered trapdoor could be a bug.
To clarify- image 2 is on edge of block with trapdoor up, image 3 is after you have fallen off the edge of block.