Is that number actually disappearing? It should be showing negative numbers if it goes below 0.
There’s only one way I am aware of to place tiles on negative indexes, which is when you have set your map to infinite. An infinite map can be expanded in all directions, but its origin does not change. So if you expand the map to the left or above the origin, then you’ll be editing chunks in negative coordinate space.
But are you using an infinite map? Since the number seems to be actually disappearing, it could be that you found a bug.