There are NO spider mites at work. No webs, no spots. I would have these by now.
There is no powdery mildew, although I suspected the same.
It is also not overwatering - these plants have only been soaked due to flushing, and this was only twice in their lifespan.

It's ok, guys. My "local specialist" (employee in a Hydro shop) has diagnosed by looking at a leaf, along with 3 coworkers.

This is rust fungus. Some may want to visually document the problem by taking my pictures as this is a rare occurrence I could not find any info about. (Look up Plant Rust in google images - you won't get cannabis leaves, but you'll see that the symptoms are the same, regardless the plant)

I'll give more word on the recovery, which starts with using a mild sulfur foliar spray.

I stress documenting this and using my pictures given the problem is very misleading and may force the grower to take useless measures (insecticide, no watering, nutes, etc) without addressing the ongoing problem.

Of course, if it is a misdiagnosed problem (wow, I'd be surprised), I will promptly update this post (with a tad of shame, of course)