Minecarts when they are occupied with any mob (villager, endermite, etc.) is getting controlled by the mob itself.  The endermite, moves in whatever direction it is tracking when on a rail.  Before 1.16.0 there was no movement from the cart. I’ve had trouble moving villagers a few days ago & now I’m having an issue keeping an endermite still on one block but it keeps moving the cart on it’s own.