Solid blocks such as stone are not displayed on filled maps, if there is no block below them.

I wanted to update a map item (Minecraft 1.13-pre 4 --> 1.13-pre6) on my server, but after I picked it up and looked at it, I could see (on the map) through the roofs and floors of my buildings. But the furnishings of the different floors were visible, they were build on the floors so they had a block below them.

This bug appeared on all the map items I updated or created.