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.
Thank you for your report!
We're actually already tracking this issue at MC-146855, so we will resolve and link this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Also, we don't mind people giving links to videos and worlds for reproducing bugs in the game. 🙂