https://www.growweedeasy.com/cannabis-plant-problems/manganese-deficiency.

Let me guess one of your auto's? I had the same problem in mine. Just a shot in the dark but I think Mangsnese def. Your tap water has a lot in it. From what I read and researched on. https://youtu.be/BpwxA8-jZTk?si=0yZO2OAEiD1LdtIl

I think you are in  Peat for your medium so Your closer to Hydroponics than soil if you will.

not saying this is it but this is as close as I got with my knowledge . If someone else has something let us know and if you can please give some articles or vids  with the answer.  I am learning as well on this.

funny thing is I have yet to have this happen on a photoperiod. Just my autos were doing this.

 If this is not it the videos are educational in themselves . I know you go by a feed chart but it is important to note that everybody's environment is different so that's why we document our feedings and problems . I do this because We may need to make different adjustments per grow and if I adjust wrong I have a paper trail to look back at. Like what was I feeding when I noticed the problem, Like I said I am no expert ,far from it.I don't think it is a cal/mag def. The reason for this I tried lower and higher amounts of cal/mag and no change with the brown spots. I noticed that I did not have this problem when using distilled water  just my tap water hince the manganese in tap water. 

If this is not the problem you can at the least learn about nutrient uptake and manganese def. 

I will just throw this out there in my experience with a medium other than water , I use to flush all the time I had a problem and it did not suit well with my autos made them worse so I am not a fan of flushing. it is always the last resort for me. I mean a desperation move. lol.