Minecarts do not retain NBT data when placed. Affects Furnace/Chest/Command Minecarts spawned in with commands. Below commands illustrate the issue.

When a Furnace/Chest/Command Minecart is spawned in with commands, say,
/give @p minecraft:command_block_minecart 1 0

the Minecart is properly created with the NBT tags (confirmed with the debug tooltips/NBT Explorer). However, when placed, these NBT tags are seeming erased or reset. For the given command, the Command section is blank.