Seeds can be fully mature in 3 weeks with some strains. You may have a plant that's genes set it up to be fully mature and starting to drop seed 3-5 weeks into flowering? It's early maturity can be a great trait if you ever want to move to an outside grow
Sativas seem to be set up to drop seeds easily. That calyx splitting looks normal in comparing it to memories of the pure sativas I grew in the 70's. With today's designer strains it's hard to know what traits may be lurking in the family tree.