Worldmap Question: How to Keep Tilemap State when exiting?

[WeLuvGoatz]

When I am in a custom worldmap, and I make a tilemap that fades in/out, I cannot figure out how to make it stay that way. When I exit and return, the tilemaps have reset to their original state. How do I fix it to where they stay in their current state even when I exit and return to the worldmap?

[manuel]

This uses the same mechanic the forest world keys use to remember whether they got collected.