In any round of Bloons TD 6 I play, all of my magic monkeys with zero upgrades can pop purple Bloons, which are supposed to be invincible against magic. Is there a monkey knowledge point that revokes this? Is this a glitch?
Purple bloons can't be popped with fire and energy based attacks unless you use Monkey Intelligence Bureau from monkey village. Not all magic monkeys use fire/energy based attacks like Ninja Monkey and Super Monkey (without laser upgrade).
From all magic monkeys with 0 upgrades, only the wizard monkey can't pop it.
You can check the wikia for full list of which monkeys can't pop purple bloons. https://bloons.fandom.com/wiki/Purple_Bloon