Yeah, when tiles are added to a tileset that is not the last tileset referenced by the map, Tiled can still load a map because it stores the “first global ID” for each tileset, which makes tile references unambiguous.
In addition, when you increase the width of the tileset image, Tiled detects this and will ask you whether you want to adjust the indexes of the tiles on the map. This needs to be done, because tiles are simply counted instead of being referenced by their row and column.
There is unfortunately a problem when you have multiple maps using the same tileset. In order for all maps to be adjusted in this case, I think you need to have them all open in Tiled when you change the tileset image. Otherwise, it will adjust the first map you open, and then won’t ask you again, because it stores whether it did the adjustment with the tileset. So, I’m glad if this feature has worked fine for you, but there is still reason to be careful since it is tricky to get this right and it’s still somewhat of a corner-case.
Also, I am now reminded, that the tile index adjustment code does not yet deal with tile object templates and Wang tiles…