This was caused by Feared or Mind Controlled players doing the last damage to either Vortexx or the Deranged Xan. This issue was fixed in code and patched out as quickly as possible. I am sorry it affected so many players before the root cause was found.
Should this issue still be a problem for either Vortexx or 12-man I would be very interested in hearing about it. Again we are sorry it took so long to reproduce this issue.