Enlarging 5th map to full size (#25) messes up 3rd map (#15)

Started with map#0 and enlarged to Map #5.
Continued same with Map #10, 15 and 20.
When created and enlarged to make Map #25 in new area, Map #15 was erased and became a duplicate of Map #25.

This makes no sense as there was zero overlap between what was on Map#15 before and after creating map #25.