After upgrading from Jabber plugin 1.25 to 1.26, our Jenkins server no longer remained logged into the various chatrooms (in our case, HipChat via Jabber/XMPP) for more than a few minutes. The behavior seems to be that it would connect if it had a notification to post (e.g., a build status), but shortly after that would drop offline again, and thus not be available to receive commands (!status, !cb, !botsnack, etc.) that users issued. Upon reconnecting, it would go through all commands issued while it was offline.
Downgrading to 1.25 has brought back the desired behavior, which was to have Jenkins remain in any chatrooms it has joined and thus be available to receive commands.