Sounds to me like the new rollback or hillstart feature is releasing, as it is supposed to.
Parking brake might be an option here, if the slight rearward release is bothersome, or just wait for it to release, then set the parking brake.
I'd sure contact Ford in any case, to see if this is normal behavior as a result of the update. If abnormal, they certainly need to know about it. Good luck.
2012 Focus; Maybe 3rd Time's the Charm?