How come it only affected the one plant closest to the light and all plants off same mother. What other reason could have caused. All fed same nutes in same tent only difference is the 1 plant that is close to dehumidifier?
I don't know but it is not from some tiny red LED. Is the plant the same strain as the others? From seed or clone? You say 28 days in to flower. Is that from switching to 12/12 or when you first noticed pistils forming? If it's from switching to 12/12 it's pretty premature for 60 - 70% brown pistils. What have you been feeding? Sounds like something is going on but it's not from the little light on the dehumidifier.
In my experience they 100% have caused these. Any light that disrupts whatever the plant considers normal darkness can and will cause stress which leads to bananas on plants that have the proclivity.
I guess the plants I grow are just special then. I've had a power strip in my tent for years and have never bothered to cover up the red light on the switch. It's been on constantly and no nanners. Or the heater I use in the winter. Another red light. So sometimes we're up to 2 red lights on in the tent. Oh wait, the dehumidifier I sometimes use late in flower, another light. Never any nanners. People can use some tiny LED light as a scapegoat but it's not going to stress a plant into throwing nanners. The cause is something else caused by the grower. Stress can occur from overfeeding which way to common and the cause for a good portion of all issues growers have. That's why I asked the OP if it was the same strain and what they've been feeding.
Going by the hypothesis that any light disrupts the plant then every plant growing outside would be popping nanners right and left due to the moon. I've had those solar outdoor lights in the backyard that come on at night right next to plants I was growing. Never any nanners and they're much brighter than some indicator light on a dehumidifier. They're pretty bright. It's easy for growers to blame issues and point fingers at the cause but 99% of all issues are caused by the grower. Whether they want to believe it or not is another story.