It is a PK deficiency; I had the same issue with my last grow Tangerine Dream and Critical Purple. I was running 2000 watt Led at 18 inches and during flower the lights were causing her to bud aggressively thus using a lot of PK and it pulls it from the fan leaves thus causing a chemical imbalance which caused the burn....I literally watched it under a microscope. I true test is to inspect the leaves not exposed to light and you will not find this burn. The good news is it did not interfere with the harvesting of some tasty high THC smoke.