The bug
I was hovering over soul sand bubbles while making a bug report, to then find myself dying when i went to get off of the bubbles which is next to land in a 3x2 tunnel and taking massive fall damage. Insta-death. Does not work every time, seems to only work if you can get off of the bubbles while still in the air.
How to reproduce
Make a 3x3 of bubble columns
Give yourself an elytra
Look straight down
Teleport yourself 1000 blocks up
Press space
Wait a bit
→ Once you hit the bubble columns, you die
I was able to reproduce this in 1.13-pre4, but not in the way you state.
Basically, bubble columns don't absorb fall damage like water does.
To reproduce:
Make a 3x3 of bubble columns
Give yourself an elytra
Look straight down
Teleport yourself 1000 blocks up
Press space
Wait a bit
-> Once you hit the bubble columns, you die.