Not an expert, but I think it’s Angular Leaf Spot. It seems to match at least, damage is not passing veins, looks like the underside of the leaves have white stuff on them. It does not look like there is a cure if so though, at least, not one I have found searching the internet.
At least it appears to be something that gets triggered. In theory, if a node is not under attack or heavy usage, this isn’t a consideration. Doesn’t seem to be a perfect solution as it still slows the traffic of legitimate users in the event of an attack. I don’t know the full details, but in the worse case it makes it easier to semi-DoS, maybe not by fully making a node unresponsive, but by making the service so painfully slow that users may give up on it.