Yeah, I'm not convinced it's a manganese issue as that particular deficiency will start with yellowing leaves at the base of the leaf and spread outwards towards the tip, eventually developing those brown spots... and certainly you're keeping your pH at a level where this deficiency should not appear at all. And I'm not convinced either that it's a calcium deficiency as those spots are usually much smaller than what your plant is exhibiting.
I'm inclined to think it MIGHT be a fungus of some type... septoria comes to mind but as septoria progresses, it will cause small bumps to grow in those dark spots and I don't see that happening yet. I would read up on the subject, though, as you know best how this has developed, how fast, etc. If it is indeed a fungus, you're going to want to act quickly to protect the rest of your grow. A decent article on it is here: https://www.ilovegrowingmarijuana.com/growing/marijuana-diseases-leaf-septoria/ If, after you read that article, you decide it is NOT a fungus or at least NOT septoria, then I would support m0use's answer to your question... and she makes some very good points on other issues she's seen in your grow.
Good luck!