Since you’ve potted up with fresh soil assuming that soil is cannabis friendly those issues with Gary should start showing corrections.
Looks like it was either a calcium issue from either not enough in there or due to a PH issue. Also could be some type of mold/fungus from high humidity or from foliar spraying causing humidity to be too high?
What did you use for the fungus gnats and when did you spray them? Lights on/off? Just before lights come on or after they went off or during lights on?
Fungus gnats are a general nuisance and of left to get out of control can be detrimental to the grow. Simple ways to control them. I use this as well as other things. Many suggest using DE (food grade diatomaceous earth) and ibe used that but don’t anymore as it tends to screw up the soil critters and mucks up the soil.
Once you receive your PH meter and check your source water if needed to lower PH and you want to save some money and also use something that is soil critter (bacteria and fungi) friendly use citric acid.
Only takes about 1/4 of a 1/4 Tsp of it to lower water PH from 8 down to around 6.5.
You’ll find this at grocery stores and costs like $6 and I still have the same one I bought 2 years ago but don’t use it anymore as I am growing with a living soil and natural nutrients with RO water and the living soil will regulate things. If your using chemical nutrients than you’ll need to monitor all that stuff constantly like PH and PPM (TDS) etc.