Sometimes when you get in a minecart you're not centered in the cart. Its very hiccup-y when trying to recreate.
Riding in a minecart you should be centered in the cart.
Sometimes you're off-center when sitting in the minecart.
Steps to reproduce:
This bug is very very finicky and I had a hard time consistently recreating it. I think it might have to do with getting into a minecart when its adjacent to a block.
x = cart, b = block, r = rail
r r r r r r x b
Seldom but still apparent.