I discovered this accidentally.

When trying to build an adventure map where the player has to jump into a window by sneaking (the window is 1.5 blocks high), I discovered it wouldn't let me into the window. I built a smaller version to demonstrate the bug, and recorded footage of it occuring.

The video was over the size limit for attached files, so I have the (unlisted) YouTube version here: https://youtu.be/KMLug8QcH9s

Hope you don't mind.