Whem you break a Minecart with any tool (Hand, Sword, Axe, Ect.) from a rail or not the Minecart will be with no 'Minecart' name and will be 'entity.MinecartRiedable.name'. When you craft the 'Minecart' Item with crafting table the name will be normal. But when the entity 'Minecraft' break his name will be buggy. Also when you break 'Minecart with hopper' or 'Minecart with chest', ect. the dropped Minecart name will be buggy again, but the Hopper name or Furance name, ect. will be normal. I hope to you fix this issue soon (Minecraft 1.9.1 - Full Release or ect.)
Have a nice day!