The bug

The NBT attribute "CustomDisplayTile" allows displaying a custom tile in a minecart. For example, when typing the following command in the chat:

1.12.x:
/summon minecart ~ ~1 ~ {CustomDisplayTile:1,DisplayTile:"minecraft:wool",DisplayData:4}

1.13 (expected):
/summon minecart ~ ~1 ~ {CustomDisplayTile:1,DisplayTile:"minecraft:yellow_wool"}

and a yellow wool block should be displayed in the minecart:

It works in Minecraft 1.12.x (i.e. 1.12, 1.12.1, and 1.12.2), while it does not work in Snapshot 18w08b. In 18w08b, it seems that CustomDisplayTile is ignored, and the minecart is summoned without displaying any tiles. Not sure whether or not the attribute name CustomDisplayTile is renamed, or just not implemented yet.