First symptoms of a Calcium deficiency. Can come from many factors. Control the EC level of the soil by checking the drain, it can be curbed by an excessive amount of ammonium, potassium, magnesium and/or sodium in the root environment in some cases.
Have you considered spraying for thrips and mites i had something similar end up happening with mine and it ended up being thrips so I sprayed it with mammoth cannacontrol. Not to say it's not a deficiency just another possibility if you rule that out and keeps spreading.
Hi, I had the same recently. Just turned out to be splash spots from watering that then act like a magnifying glass and burns these spots on the leaves. Won’t cause any lasting damage. Just be more careful when watering.