Damping (fungal issues)

A customer has a question and I hope we can get some opinions on it, thanks

“A problem I am having is damping (fungal issues), so far only one of my five seedlings are healthy, all others have grown to about 1.5” in height then had their stem go brown. BTW I am not over watering. You can see the stem go brown compared to green plant treated in exact same way, fear fungal problem has killed three seedlings.

Seed: Auto flower Northern Lights
Organic Soil Mix (with worm casing) in 3 gallon grow bags
Nutrients: Too young for nutrient adding
Lighting: 10 Hours natural light (outdoors), 12 hours fluorescent 2 hours dark
Constant temp 76-80 F Humidity 89%"

No offense but I hate those type of pots they are misleading. Maybe just transplant to the big pot?