Well, there are two orders in which you can do the turn on, turn off of the FoW, and you might get different behaviour.

1) Spawn Army
Turn FoW Off
Turn FoW On

2) Turn FoW Off
Spawn Army
Turn FoW On

I'd figure the second way wouldn't cause any problems, because there would be no FoW to 'swallow up', and then putting it back 'should' not cause any problems.