WARNING: I am not an experienced grower but am happy to help brainstorm what it could be.
To me it looks like a Mg or Ca deficiency. I saw that you are adding Cal/Mg, so it's not a lack of trying, it's just being locked out. Are you using RO?
Based on the pH range you are running (mostly 5.8-5.9), you might be locking out the Mg/Ca due to the pH. Take a look at these two charts. Your running right on the edge of where both elements uptake is impacted. Maybe try raising it up to 6.3 or down to 5.5 for a couple days and see if the brown spots go away. If so, then maybe you can just occasionally raise/lower your pH so the plant can uptake the Mg/Ca (doesn't need much) and then get it back to the pH you think is best for the other nutes.
You could also consider keeping your pH where it is and foliar feeding the Cal-Mag (ask someone more knowledgeable than me on this) to test if that is the issue. Based on your humidity, the leaves should be very willing to take in the nutrients and the root level pH won't impact the foliar feeding.
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