What's the deal with FRLG hidden items?

I had the exact same problem when I could not find the TinyMushrooms in Mt. Moon after starting a new game and I could not find an explanation anywhere on the internet.

I started asking questions around and someone finally helped me on the Pokecommunity. Here is the link, Basically it says that hidden items can be set dynamically, so there is an in-game event that sets the ability of picking them up. This event can really be anything, for example:

• Acquiring Cut or the second Gym Badge
• Traveling through Diglett Tunnel

Maybe we can find out in the future for all hidden items. It baffles me that after almost 15 years (!!!) we still don't know when the hidden items spawn in two of the most played games on the Game Boy Advance. It's too ridiculous to be true, but, it is true. No one knows it.