Map editing: List of unused tilemaps?

And thanks by the way for looking into this, I really appreciate it! If you need any more information, I can also send you my game.

I did the same tests but with map $1A0 I gradually increased the map size up to 2048x1024, drawing tiles near the bottom right corner of the map and saving/closing the editor each time. There was no issue. I could look at your ROM if you send it via pm along with your settings.xml file.