Map editing: List of unused tilemaps?

The highest tilemap number in the editor is 15E, and 15F seems to work okay, too, but going higher than that led to an issue of not saving the map correctly. I tried tilemaps 160 for Layer 1 and 161 for Layer 2, but after 4 hours of map drawing and saving, I had to find that my new map was gone after reopening the editor. I never got a warning message when increasing the tilemap numbers, nor while drawing my huge map. I doubt that I exceeded the available space, as this would normally produce the warning message informing you that it didn't save, so I'm not sure what's going on.

I used a vanilla ROM and made the default expansion. Then on map 0x19F I used tilemap $160 (L1) and $161 (L2) and draw some tiles on each layers. I saved the levels then saved the ROM and when reopening the editor and re-loading the ROM my changes on the new map were still there. I did the same with map $1A0, this time with tilemap $162 (L1) and $163 (L2). I have not modified any map property and I used the default tileset. I also did not test the map in-game.

Do you have more details regarding the saving bug?