I got this issue a week ago. My plant is 2 months (veg) and 2 weeks (flowering) old. There is no bug around my plant. I feed my plant with Dyna bloom, 1000ppm. Run off is also 1000ppm. Ph is 6.5. I did flush my plant before flowering. I can't figured out what the issue is. Pics:
Just to rule out mites, did you check the underside of the leaves with a lens? Can't be too careful because mites can quickly get out of control
I just checked the underside, I saw no sign of bugs. The issue started from the bottom of the plant and it's moving up, but new leaves are still ok.