When you teleport a minecart with a player inside to a place outside the current loaded chunks, it gets stuck with a "continuous back and forward teleport", but player coordinates are still in a point between the origin and the desination of the teleport. You can't get off rhe minecart and it doesn't exist (/kill doesn't find a minecart) but if you teleport the player, he will appear to relative 0 0 0, he will appear in the origin point.