Closing a trapdoor on half of your body doesn't lower you

In the latest beta, the changelog states:

"Players will transition from swimming to sneaking if they are unable to stand up but would be able to sneak"

However, this does not appear to be functioning correctly, as using a block such as a trapdoor or a big dripleaf to push yourself into a one block gap does not work. Instead, it either does not do anything or pushes you out of the block.