Brown Spots Diagnosis

Looks like a calcium symptom. either not enough is provided or something is locking it out. pH is fine, so that eliminates one of the possibilities. You have a N-deficiency occurring too. If light were too intense there'd be other obvious symptoms to verify. I'd adjust fertilzer formula -- keep everything the same as best you can while increasing Ca some increment... see how it goes for a week or two and adjust further if necessary. Existing damage will not heal in this case, so stoping the progression is the visual cue it worked. This stuff doesn't happen overnight (too often). The slower it progressed, the smaller the adjustment needed. 25cm is quite close, but depends on umol/s produced from the light and how well it is distributed. There are advantages to using a bit mroe power and greater distances from canopy. You get better distribution and light penetration, but at some point it is definitely wasting more watts relative to incremental benefit as you hang further away. Light spreads out based according to inverst square law. Consider distance to canopy is the 'increment.' Doubling that distance is 1/4th the light from a single point source. So light is 1/4th power 10" deep or 18" deep - if the hanging distance was 10 vs 18. you can see how this helps deliver stronger light beyond the canopy. At some point the benefit is not worth the cost of course, but it's probably further away than 10"

What is the RH% at night going up to in that little tent, 81F 65%, if that's daytime, I hope you are dealing with night RH if it goes as high as I imagine it would the plant will not perform cellular respiration as you would easily be sitting at 75%-80%+ overnight. The plant needs a gentle breeze at night along with an exhaust fan keeping humidity somewhere 40-45% at nights, doing so would keep your VPD somewhere in line with your daytime and nighttime temps, creating a good VPD with correct negative pressure, this will do wonders for your water cycling through the plant, faster you can cycle water, faster you can get more nutes to where they need to be, right now production seems to be hampered, those long fabrics are not as airy as I thought. If water doesn't evaporate or uptake then it just sits in the pot in high-humidity environments the water in your medium will stay a lot longer, by keeping your air around 40-45, the moisture content in the medium will slowly wick to the air preventing moisture sitting in the pots possibly causing rootrot or whatever. You water frequently but the production line is crippled by being limited by transpiration at night, if the plant cannot eject its water then it cannot uptake any more nutrients. As you feed and feed, add more mineral salts to the medium, but the production line is down so nothing uptakes. I think I butchered that, but I'm high as a kite anywho so good luck!

Looks like calcium deficiency also a bit lack of nitrogen to me