On the Isle of Armor, according to NPCs, you need to do Max Raid battles in order to cause Max Mushrooms to grow. However, it's not clear if you need to win those battles, or if you just need to try them.
I ask because I have done several (4-5) Max Raids, lost most of them, and only found a single Max Mushroom in this time. I search all the caves and the forest between raids.
Best Answer
I think it is possible for mushrooms to spawn if you loose, but I'm not sure. I tried to beat a 5 star raid many times (maybe about 10 times), and when I gave up I found two max mushrooms. I did win a bunch of easier raids before that and even though I think I checked the spots before the 5 star battles I am not 100% sure they were not there.