Yes, this is a duplicate but before you address it as a duplicate, please read on.

Easy to recreate

I have confirmed that when you sneak on a chest or other non-full block and you walk to the edge, you fall down instead of hanging over the edge on 1.6.2 and higher. This is particularly annoying whilst doing parkour, because you're close to the end and you fall down when you've done nothing wrong.
This also happens with half-slabs and the bottom half of stairs, but I can see why they stopped it with stairs.

I presume this happens because the hit-box is significantly different to the actual box of the chest. This confuses the game and, therefore, doesn't keep you on the block.

Attached are 3 photos of me trying to attempt it.