GroupWise 12.0.1

UserA has posted appointments in his calendar. Those appointments are
showing up in UserB's calendar....sometimes. UserB saw them, then
they were gone. Then UserB switched to multiuser calendar (UserA and
UserB displayed) and the posted appointments again showed up on both.
Switched back to single calendar and UserA's appointments were still
listed on his (UserB) calendar. I'm stumped. How is this even
possible? Any clue about what is going on? Or how to fix it?