Built-in editor and #defaultmapzoom. Bug.
 

I made a tiny map with the Random map creator and saved it. Then I opened it with the built-in Map Editor and saved it.

This changed the map file headers slightly:
#domversion 306
#defaultmapzoom 3.981458
is replaced with
#domversion 175
#maptextcol -1.00 -1.00 -1.00 -1.00

Removing the #defaultmapzoom line makes the map filter icons way too big in the game, the temple symbol covers half the province. It seems like a bug that the line disappears, and I haven't seen it posted (though it probably has).