I also chased it thinking it was this and that first at first, Cal than mag then nothing helped and plants with dropping dead everywhere then I got my humidity in check and it all stopped
nothing us lacking, just never had to seal the room this early not even close but it's been nothing but rain and cold in New England particularly on the coast so looks like an early start :-/
.. here's another picture, classic septoria (early)
View attachment 4149759
Waters 104 PPM I'm using GH floral Trio in ffof, supplement with light calcium, than magnesium on a fairly regular basis, if not every feeding. Sealing room today, may keep that plant around not sure yet, not finished trimming it
* the white specks all over it is baking soda spray