I had a batch of 6 that got it during flower. I would carefully remove the infected leaves in hopes of keeping up with it. In the end, it spread faster than I could keep up and I ended up cutting 3 plants early. The other 3 were completely resistant. It was due to inadequate air flow. I hadn't trimmed the lower parts of the plant and it became a jungle. While the room humidity was fine, down inside the jungle it was much higher and the PM took hold.
It's like herpes - once it's there it's not going away. Air flow and lowering humidity will slow it but not stop it. Treating it with anything will kill the outbreak, but it will be back. The buds are a rich source of moisture too - once it gets to them, there really isn't much you can do.