I had this issue recently, I thought it was heat stress, I investigated once my night period began. I noticed Thrips! They suck sap from foliage and other plant material, culprits of my (spotting on my foliage ) issue. I mixed distilled water with soap. PH this to 6.5-6.8. I used a cotton swab to apply the mixture on the top and underneath all foliage in your grow area. I also applied my neem solution(6.5-6.8) to the surrounding area (meaning any area pest could hide..i.e...containers humidifiers, etc.
You can look up neem solutions and soapy solutions online, let me know if you need a recipe. Good luck growing!
note: The solutions I mentioned will assist in the immediate removal of any soft body pest when applied directly to the underleaf area.