Unity update does not be called when lock screen

Hi Guys, I'm a game developer working on Unity 4.3. Our online game has been released and there are lots of guys playing it. Since our game supports auto fight, the player can just leave the game there and join a party, After he comes home, wow, he will find he has defeated lots of monsters....

But now we have found a Unity bug which does not let us do that. We just find Unity 'update', 'fixedUpdate', 'LateUpdate' function will not be called when player lock the Screen! You know our auto fight and network communication logic all rely on the Update loop, So if they are stuck, our auto fight logic will be impossible to achieve.

As I research, I have found 4.1.2 will call Update even when lock screen by default, but 4.3.4, 4.5.0, 4.6.6 will not call it. Fortunately, Unity 5 also support call it, but you know, our project is a really big, so if we upgrade it to Unity 5 or downgrade to 4.1.2 both means a lot of work, and player has to get a big update, which is definitely not what we wished, so I'm just here to post the question and try to get some help from you guys.

In fact, I'm not quite sure if there is a setting which can set this in 4.3, if you know, please tell me or some other possible solutions. I'm really appreciate here ^_^.