Do you have "Auto-Join" enabled or disabled?
No IAP in a cluster is designated as a permanent Master / VC. If the active VC goes offline another IAP will automatically take over as the new VC and will remain the VC until it goes offline / reboots. This is, in part, based on system uptime.
That being said, that IAP should re-join the group as a member so something is wrong.
What version of SW are you running? v.3.3 is the current and has a number of fixes for similar issues. Without TAC involvement, however, I don't know if you are hitting one of these issues but this is an easy thing to try.