From latest release notes:
NEW: The raid now has a lockout mechanism to prevent characters from entering the raid once the Artifact has been activated.
NEW: The raid now has a lockout mechanism preventing characters from entering the raid once players reach the Shrine room.
Unless you've fixed the total lag-out where every single player freezes completely and is unable to take any action, while the mobs stay active and kill us all, there should NOT be a lockout mechanism.