When using any map in 1.13, any and all blocks at the block level Y = 0 will be ignored from the map, and instead the void will be displayed. Every block above Y = 0 is unaffected.

This occurs in every type of world and with every level of map.

In the End, outlines can be seen, but they lack any colour. The Nether is unaffected because it doesn't display any information.