Double Digit Maps erase info on earlier maps

I had made a lot of maps, and when I started getting into the double digit maps, my previous maps started having all the terrain info erased from them one by one.

What I expected to happen was...:
All maps should work as normal

What actually happened was...:
Each time I created a new map with a double-digit map number, it would erase all the terrain info off of the first map created (that still had info on it).

Steps to Reproduce:
1. Make maps 0-9
2. Make maps 10+
3. Start losing data