I summoned a commandblock_minecart in superflat world.
However, When I opened the world next time, it disappeared.
It can't be detected by the command "execute if entity @e[type=commandblock_minecart]".
So I think it did really disappear.

This problem can be reproduced by summon a commandblock_minecart and exit the world.
While loading the world again, the commandblock_minecart disappear.

Other kinds of minecarts are display correctly. Only commandblock_minecart is wrong.