I can confirm that the mushrooms improvement/bonus is not appearing when the event triggers. I got it on a hill, but not sure if it is supposed to trigger there.
I wonder if the mushrooms improvement will work better as a bonus rather than an improvement?
I've got another bug here, as Ljhosalfar, you can't chop down forests or jungle, even with the prerq tech, the button is not even there. I know you made it so that the AI would recognize that it won't destroy forests, but why remove it for the player?