Users disappearing from groups

My Openfire is 3.10.3 running on CentOS 6. Clients are Macs running Yosemite 10.10.5 & Apple Messages. Chat works just fine, other than if a user right-clicks on a name, “Create New Group with People”, enters a name, and OK, a group is formed with that person in it… but after a minute or so, the other person disappears and returns to the default group, leaving the newly-created group empty. This does seem more like a client issue than server, but I’m hoping the experts here can give some ideas!

The only advice i can give is to not use Messages. It is known to have bad xmpp support.