Hi, I discovered what is the likely cause of this. The map looks OK, editing on JOSM looks fine, but with iD inbrowser editor some phantom water area borders can be seen near Kopališče Kolezija and FGG. In josm, I see a validation error: "Incomplete multipolygon relation". If the polygon is incomplete, I bet the phantom lines are just a line from one end to the other end.

I removed the fixme, but I'm not sure what to do with the incomplete multipolygon.