Yellowish leaves with brown stamps

My first thought was environment as you've got some cupping which is the leaves turning in.They're trying to protect themselves.Are you using LEDs? as they need heat due to spectrum.If its not hot enough you can get bleaching.If the LEDs are strong then they need to be over 27 but 24 seems to be enough if its under 600w.I experienced it a couple of times after switching from HID lamps.They will use more PK in the first 2wks of flower and will take it from the leaves if it isn't provided.They also need more PK in wks 4-6 with an 8wk flower.

I would recommend feed normal but add 5ml/gal of some CalMag and it should prevent your issue from getting worse. The brown spot that are there won't go away but your plant should recover just fine. I feed CalMag every feeding with the same amount I recommend and never had any issue from it. I hope this helps and your plant gets better. Happy Growing

Looks like magnesium deficiency - right location, interveinal chlorosis with rust spots. you could top-dress some generic epsom salt on the plant showing the symptoms or mix it into current fertilizer. skinny leaves in new growth might be too much p or k. i don't think that's the case here. are they possibly a bit too close to the light? if only impacting the tallest locations, might be a reaction to that too.

Without a diary to look back to, it's kind of hard to come to a conclusion. How many plants are there? How many of them are affected? What is your watering and feeding schedule? What is the ph of the runoff? How is the humidity? So many questions left unanswered 😓