So I can't figure out what causes it yet, just haven't really tried, but I tend to use /focus <primary character>. Then /assist focus to attack. This way I can /cast [target=focus] heal, and keep the target to support DPS... Anyways I'm finding I lose my /focus at some point while playing. not sure if it's zoning, or after a certain amount of time. I'll see if I can track it down. and let me know if anyone else is noticing this or not. I do know prepatch my focus target stayed the same the entire time the character was logged on. You would only lose who your focus was upon exiting the game. Not just logging out, but actually exiting WoW if I recall correctly...