Summary:

If a minecart sits on top of a block that is y = 31, minecart itself being y = 32, after exiting and reentering the world the minecraft  sinks through that block. Tested on y levels 5 - 255 and occured only on y = 31.

Steps to reproduce:

• Create a flat world
• run /setblock ~ 31 ~ stone
• run /summon minecart ~ 33 ~
• Exit and reenter the world

Observed result:

Minecart sinked through the bock and it is on the ground now.

Expected result:

Minecart staying on that block.