The boffins have told me it will be fixed in MCD6 SP1 as mentioned. Apparently to be released at the end of May.
We've been having Hotdesk ACD agents failing over to the secondary, then after the health checks they fail back. Once in a while the hotdesk login goes into lockout state instead of failing back, so have to issue BUSY RESET <DN> to fix. Apparently they think it's a voicemail cleanup process consuming too many resources. Our failing site is on 5.0 SP2 PR2, although they acknowledged the problem is in current MCD6 releases.