A block that was already mined still had collision and killed me.

I made a water elevator which allowed me to drop down the sides using gravity or climb up the water. I used both functions several times without issue.

 

However, I tried dropping and holding forward and died by landing on the collision of a block that had already been mined and shouldn't have had collision.

 

https://www.youtube.com/live/wD4KigiD0uU?feature=share

I have the incident, several reproductions of, and a fix for the incident linked starting at 11:05.