Cartographers generate maps for existing Mansions and Monuments rather than unexplored ones

Cartographers will always unlock maps to the nearest woodland mansion or ocean monument, regardless of whether or not the chunks/regions have been explored. 

In 1.12 whenever a map would be unlocked it would always lead to an unexplored area. 

This is especially problematic for servers.  (Especially larger servers) .

The map size for generated maps may be too small.