It's beerstone. It is always gonna be a thing. It gets worse the lower calcium your water supply.
Barkeepers Friend works. Hot strong PBW with sustained contact does too. Nitric/phosphoric acid blend works best (it's what pros use), but that's harder to get and more dangerous to use at home. For home use a very hot PBW soak and then hitting quickly with an abrasive scrubby makes short work of beerstone.