I think it might have to do with global/local coordinates.
If you design for e.g. full hd and your clicks always result in global coordinates they might cooincide with the local coords because you started from 0/0 with the full hd viewport. Your game does not stretch on resolution change but rather expand. so the click coords (global) cannot be used to get a tile from the tilemap, because it needs to be converted first.
tbh this is from the top of my head, as i fiddled with tilemaps myself some time ago. do not know if this is the actual problem at hand.
edit: look at the docs here: https://docs.godotengine.org/de/4.x/classes/class_tilemap.html#class-tilemap-met...