In the paper More is Less: On the End-to-End Security of Group Chats in Signal, WhatsApp, and Threema, the abstract claims that
We additionally show that strong security properties, such as Future Secrecy which is a core part of the one-to-one communication in the Signal protocol, do not hold for its group communication.
Failure of future secrecy makes sense for Whatsapp and Threema, since Threema uses a static key and Whatsapp uses a single symmetric key ratchet for group messaging, both of which are vulnerable to session state compromise. However, the paper states that Signal uses a normal double ratcheting scheme for group messaging such that group messages are indistinguishable from pairwise messages.
How does future secrecy fail for group messaging when the session state is refreshed whenever the keys are ratcheted?
Note: It seems that future secrecy fails due to the ability to join a group just by knowing its GroupID. Assuming that group update messages are properly authenticated, does future secrecy still fail for Signal's group messaging?