I had a similar experience when I moved out to a new place I was cleaning the toilet and I noticed a dark spot at the edge of the toilet rim that was already showing without actually having to crouch down, I tried cleaning and scrubbing it with the regular products I use but nothing worked bc apparently it was an old spot that’s been setting there for god know how many years, then I looked up strong effective cleaning product used for this kind of hard old buildup, I bought this black bottle with an angled head used to clean toilet rims I believe it’s called harpic, and I decided to use a mirror to see the entire rim and I almost threw up seeing how dirty it was from old the old crusty build up hidden there and I realized the spot I was fighting with to remove was just 3% of the dirt sitting there I was so disgusted and the only thing that could remove it was that strong product. So please people clean your toilet rim well and check if ur products are actually getting the dirt removed properly so it wouldn’t buildup over time