Not sure if this is the way it is intended...

Pre-combat magic seems to be cast before "turn 0", even if the army of the mage casting it is surprised in an ambush. This could affect the army winning the tactician's round, though they should have a "free turn" to attack...

It would seem to me that the ambushed army shouldn't get to cast pre-combat spells until after turn 0, just before turn 1, when the army is recovered from the surprise attack.

The easy answer is of course that "mages have a higher awareness, and are therefore not surprised, but don't have time to warn their comrades"...