Literally methodically searched the forest with a map and cannot find the 10th mushroom - are we being punked?
The 10th mushroom does indeed exist. It's toward the center of the forest. I had a bit of difficulty finding it myself because I didn't think I could enter the screen it turned out to be in. There's no dead space in the forest map.
Do you recall what it was next to? What screens? I literally made a map so I may be able to find it from there. Or perhaps I already have!
It's possible we're not talking about the same 10th mushroom. In my map, I marked an M in every screen I found one. The last one was almost perfectly in the center of the map. I'd suggest restoring to an earlier game and marking where the mushrooms are on your map. That might make it easier to identify the last one.
Maps
https://steamcommunity.com/sharedfiles/filedetails/?id=1640439385
Your map is really nice! I see eight brown mushrooms marked. The two which aren't marked are at "one dead tree" in the lower left, and "1 tree 1 log top left" at the upper right.