Dawn reported this over at wowinterface.com. Here a link to the forum thread there http://www.wowinterface.com/forums/showthread.php?p=230195

This is because the player and pet frame switch units. The frame which was the pet frame now holds the pet unit, and thus is the owner of the AltPowerBar for player.

If you don't have a AltPowerBar set for that unit, then the player AltPowerBar will go void.