If you directly summon a minecart With any passenger (panda, wolf, marker entity etc) will summon the entity causing the Minecart to be occupied, When modifying the NBT of minecart using the /data merge entity command by adding Passengers (Any entity) causing the Game to show that it has modified the data of the minecart but no passengers appear, this is a Inconsistency for NBT data even if its super small.