If there is more than one minecart, only one will show up.

Yeah, currently duplicate entities show up as one tab. That is part of the reason why 0.9 is currently a beta.

Block tabs use their coordinates to handle duplicates, but I can't do that for entities since they move. I think I have figured out a way to fix this, but I won't be able to push the code as I am going to be very busy for the next week