As far as I recall it always worked this way.
a) If you kill enemy, you don't lose stealth and other enemies are surprised and don't move until your next attack (you can even move all your mercs, it is tactical mode already).
b) If you fail to kill enemy, you lose stealth and they move instantly.
Advantage of initiating combat with Fox is that enemies don't move even if she does not kill enemy and thus loses stealth.