This was a deliberate design decision by WhatsApp. I even remember going to a talk by a WhatsApp engineer when they announced end-to-end encryption and he spent

This was a deliberate design decision by WhatsApp. I even remember going to a talk by a WhatsApp engineer when they announced end-to-end encryption and he spent a fair amount of time detailing the method WhatsApp had implemented to allow for adding new users to groups while allowing these new users to read old messages. So I'm pretty sure this has been known from the start. Don't use Facebook products if you care about real security.