To try and test this out I created a new Sculptor page. I made a couple of edits to the page then clicked on finish. I was prompted to save a preset, so I made a new name and saved the preset. I then went and created a new Sculptor page, this time I made a single change to the new preset that I created. When done I clicked on finish and when prompted to save a new preset I just saved over the existing one I had created. I was then prompted to confirm overwriting the original preset. When I went to create another Sculptor page I checked the areas that I had edited and saw that my edits had been retained.
If you could give me a similar series of steps that you are taking that make it so the preset is not overwritten I would then be able to test this out to see if I can reproduce the result you are experiencing.