Lowering height by one block causes fall damage even with water brake

Jumping into the temporary hole on the main floor of my new house at level 88 works fine, safe to mining level 10. From the basement at 85 to 10 is fatal.

https://youtu.be/SMbzFgMnI48

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Original description:

Dig a 2x1 hole from y:66 to y:11 straight down and install ladders, adding a torch every 10 blocks on the opposite wall.
Place a sign opposite ladder wall with two open blocks beneath it.
Place water above the sign, creating a water brake.
Test. Voila. Works very well.
Now lower the height of the hole by one block, removing a ladder block, so you how have a top floor at y:65.
Test.
Every time I now take significant fall damage - aka death. I've tried many things, even holding the backup key to stay glued to the 'torch wall' all the way down but I still die, with the water brake in place.