This mainly seems to be a visual glitch because the hitboxes seem to align normally but carts inside of bubble columns do some funky things.

Example imgur video 

I think the carts should only tilt there is lateral movement while falling instead of always tilting while falling even when straight down. Or maybe some special rules for water and/or bubble columns because I use a lot of bubble columns to move carts around.

Steps to reproduce:

1. Build the following setup in a world with the Minecart Improvements experiment enabled:
   
2. Place a minecart on the rail and push it into the water

Observed result:
The minecart stays tilted even after it hits the top glass block.

Expected result:
The minecart stops tilting after it hits the top glass block.