I know the internet is full of fear-mongering. “Brown spots mean CANCER!” (No.) “They’re mold!” (Probably not.) “The plant is haunted!” (Only on Tuesdays.)
The truth is boring and wonderful: brown spots from guttation, thrips, or cold are cosmetic only. They’re like a chipped nail on a perfectly good hand. You cook that bok choy, and the spots disappear into the stir-fry. It’s magic.