The October 6 patch fixed late join errors. Among them are incorrect display of the skeleton and individual cases of the Medieval History icon being unavailable after rebirth.
Teams that are not meeting at the same time should check the version number before the next lift. The developers specifically mention problems with a slow computer or unstable connection, but this does not mean that all causes of delays have been eliminated. If the unusual behavior persists, record at what stage the participant connected and what others saw. This scenario is more useful than the general message “the co-op is not working.” The patch is a fix for specific cases, not an announcement of a new map.
Sources and checks
Patch Notes 2.6.b · Steam