An unnamed Minecart with Command Block changes its name when reloading the world.

Reproduce:

1. Take an unnamed Minecart with Command Block "/give @p minecraft:command_block_minecart"
2. Build a track like the one on my screenshots
3. Enter the command "/say test" into the minecart
4. Let it go
5. The message in the chat will be "[Minecart with Command Block] test"
6. Leave the world and re-enter
7. The message in the chat will be "[@] test"

A Repeating Command Block with the command "/say @e[distance=..2]" placed on top of the Redstone Block confirms that. The name will change in that message as well.

When breaking the Minecart with Command Block in Creative mode after re-entering the world it wil drop a Minecart named "@"