Feminized plants do not make seeds on their own, which surprises people who find a few in their jar. Seeds only form when pollen reaches the flower, and the source can be an intersex plant in the same room, a missed male nearby, or a neighbour's garden. One of the answers at https://queenfeminizedseeds.com notes that cannabis pollen is microscopic and travels a long way on the wind, including from feral hemp. Outdoor growers should treat a handful of seeds as a pollen problem before blaming the genetics.