Searge says:

Fixed for Minecarts. On Itemframes this behavior is working as intended

Despite the following entities' NBT data having CustomName and CustomNameVisible set to proper values, their nametag remains hidden

ItemFrame
MinecartRideable
MinecartChest
MinecartFurnace
MinecartSpawner
MinecartTNT
MinecartHopper
MinecartCommandBlock

A check using /entitydata shows all of them have the tags properly set when the issue occurs, but the nametag still doesn't show.

All other entities seem to work properly, it's just these that don't.