I'd like to digitalizing a map and when I draw a new polygon which overlaps an existing one, at first it merges the boundaries correctly, but after i'm saving the layer edits, the new boundaries overwrite the old ones. I enabled topological editing. Anybody knows why does the QGIS do that?
I presume your
Avoid Intersections option in the digitizing options is on. You can turn it off by selecting
Advanced in the Layer selection and deselect the needed layers.
You can read up about the topological editing function in the Editing section of the QGIS documentation.