I had a full explored map that was 3 out of for in size and when i upgraded to 4 out of 4 he completely reseted like i never explored the area with him.

The world im playing is full vanilla without any experimental toogle on theres 3 fotos below one with the both mapas in the same screen the empty one is map 1 (4 out of 4) and the fully explored is the map 1 (3 out of 4).

Note: non of the maps had its name changed by any chance.