There’s no radiation drop after just 30 years from americium 241. It has a 450 year half life. After decades electric components start to fail and\or things get dirty. After 30 years of getting smoke in it, there was probably a layer of dust\smoke over where the radiation is at that were blocking some of the radiation all the time, that made it more sensitive.

Same issues will happen with photoelectric detectors. It’s recommended to replace both types after no longer than 10 years. I have no idea where the person you responded to got the information about them not needing replaced as often as ionization detectors. If anything, it’s actually the opposite.