After every mission, Van Wynter's store changes all items  to Assassin only gear (with the exception of some belt items) after missions. It also occasionally changes to the store to Assassin after quickly tabbing over to the Void Shards tab. (By occasionally, I mean I've only seen it once while writing this and haven't been able to recreate it.)

This locks out the ability to sell her Sororitas items and requires "getting lucky" by leaving her store menu and reopening it or backing out to menu.

I also noticed that if you equip items while the store is bugged like this, your new load out isn't saved when going into a new mission or backing out to menu.

Playing on PS5 w/ a seasonal character, if that info helps.