when i added a cobble wall to the tip of my tntminecart missile the tnt minecarts suspiciously unstacked and fell off the missile. the wall was not touching the minecarts in any way (at least 4 blocks away) and the minecarts didn't unstack when there was no wall on the tip. the same thing happened when i added boats to the tip of the missile (also not touching). i have not tested it with anything else.
link for missile design: https://www.youtube.com/watch?v=RKCOuCPk8yA