Powdery mildew

So I acquired some plants that were in pretty rough shape. Root bound, lanky, buggy and also carried a slight smell of gym socks. I gave them lots of love and seemed to rid them of all ailments. After caring or them for about 3 weeks in veg, they were flipped to bloom and they seemed to be doing fantastic. Roughly 2 weeks into bloom, they developed the powdery mildew again and I can't seem to get it under control with and I have almost 4 weeks left in the cycle. The conditions in the room are dry (<40% humidity), but tight due to limited space. I am trying to determine if the mildew aver really went away, or if the room conditions caused the problem. I'm sure it could be either way, but the question is if a plant is so far gone to begin with, may have the PM stayed in the roots? I've never had issues in the room before now.

I feel ya, I'm also fighting pm. Try the milk thing to control it until harvest and pruning for better airflow.

Powdery mildew (PM) is systemic. That means the fungus is living inside the plant.
When you see an infection (powder) you are seeing the fruits / flowers of the fungus. These fungus flowers are what spreads spores.
If a plant is infected with PM, the PM almost always comes roaring back during flowering, even if no visible signs were evident during veg.
If you are only 3 weeks into bloom, I would personally start over.
If you are too stubborn you can try a product called GreenCure (it is potassium bicarbonate) to treat infected areas immediately before removing them from the garden space (to minimize spore spreading).
A hotter room (90* +) might help.
If you are outside, you might just go for it anyways.
Sorry to hear about this after you nursed them back, but I have made the mistake of trying to finish with PM, and it is not worth it IMHO.