An issue in the handling of group membership was resolved with improved logic. This issue is fixed in iOS 15.2 and iPadOS 15.2, watchOS 8.3, macOS Monterey 12.1. A malicious user may be able to leave a messages group but continue to receive messages in that group.
According to WASC, "Insufficient Session Expiration is when a web site permits an attacker to reuse old session credentials or session IDs for authorization."