Hey all, I've recently made a new save file on my FireRed version and I think I encountered a glitch of sorts.
I played through ignoring most of the hidden items I already knew about, so I can remember what I'll find and not find easier later on at once.

So I was doing some beach-combing, until I got to Mt. Moon, and I was supposed to find a few tinymushrooms/big mushroom around the BF2 areas and I came up with nothing. I picked up one already, but only one, and I also got that moonstone in that rock near the last area but... coming back here, no hidden items are detected, I even pressed A for every tile in those 4 mid-sections and came up empty. Now I'm at the underground tunnel where I should find 6 random items, but those don't seem to exist either. It seemingly works for the overworld and every other underground/dungeon/building such as Viridian forest, where I got the 2 hidden items via itemfinder.

So yeah... keep in mind this has nothing to do with items like leftovers which work directly above the item, those are fine, it's as if getting to a certain point or having so many badges simply made these items 'disappear for good'.
I looked it up on google and came up empty, I can only guess it's something to do with my cartridge aging... even though it's pretty mint and never had anything to do with AR or the like.

TLDR: Itemfinder wont detect items that I didnt pick up in Mt Moon and underground tunnel (cerulean-vermillion), plus items seemingly don't exist anymore.