Manual failover

that was about to be my next suggestion, to have a look at the preferred device order...

I just checked the Auto Failback help description on v13:

"Specifies whether the traffic group fails back to the initial device specified in Failover Order. When checked, causes the traffic group to fail back to the initial device specified in Failover Order whenever that device is as available or more available than another device in the group. Note that the initial device becomes active even when other devices in the group are more available. When cleared, causes the traffic group to remain active on the current device until that device becomes unavailable".

So it sounds like either you should try to actually enable Auto Failback, or as you mentioned tick the "always Failback to First Device if it is Available" option