For reference:
https://forums.triplea-game.org/topic/4294/france-goes-first-2-5-not-working
I saw someone mention earlier about sub hits coming off first.
I believe a DD changes when a Sub fires. With one, a sub fires when everything else does.
So I think a defending sub would fire in surprise strike, but the attacker sub would fire with other units if the defenders have a DD.
Also in the opening fire phase, sub hits could be taken on defending subs that also fire, thus preventing a unit from not getting to fire if you have enough subs to cover the hits.