For me personally the point is now moot as I've uninstalled ElvUI. It's a good UI, until you get as many addons as I use. Jamba is now back to performing as expected.

The message was something akin to [JambaTeamIsOnline] teammember1, teammember2..., teammemberN, just as you described. Something about ElvUI's chat interface is intercepting it.

If you need me to test, I still have the current WTF for that installation. Just let me know.