Septoria?

After a while and with the help of this very useful paper I conclude It was a phosphorus defficency: https://www.semanticscholar.org/paper/Characterization-of-Nutrient-Disorders-of-Cannabis-Cockson-Landis/ffe410876342682a0c8c1e4370ff7f31ed9c030a

Good afternoon, in my view, if it's just on the lower leaves, it may not be a serious problem. But yes, it could be a lack of light; you can do some defoliation and remove those leaves that aren't getting light. If it persists and spreads to the new leaves, then you might have a problem.

I couldn’t tell you if it is or isn’t septoria. However if you’re struggling to keep RH up, I suggest going to a pharmacy and get yourself a humidifier. A cold mist humidifier would be ideal. And as for the plant, get some Safers 3 in 1 spray and give your plants a little shot of that just after lights out so the plants can dry overnight. I saw this post last night and I think maybe I might have the same issues with a few of my clones so I sent ahead and shot them with some 3-1. Even if it didn’t need it, I kinda did it as a preventative measure.