Whats going on?

It does look kind of like a cal mag deficiency but more so like the beginning of a nitrogen toxicity. I'm assuming that's why you dropped the feed down to 200ppm? I think that was the right thing to do and so I would just see what if the cal mag/ dropping the feed helps. Unless this was your ppm before this started happening? Then that itself could be the issue. Hope this helps.

the light isn't the issue, the first light burns happen at the top of the plant. anyway, you need to correct it with cal mag, I would also slightly increase sensi bloom to 3 ml. the color of the leaves seems a bit light to me, but photos often deceive... so if you see them light, increase it; if they're dark, don't.