Translated Campaign: Spitfire over Dunkirk

[gabuzomeu]

That's by mission design, bombers disappear after they bombed Dunkirk, this is to avoid overburdening the game with returning bombers and still get many waves of them.
In the mission description, this is why you are told to kill bombers before the bomb dunkirk.
Explained on Sukhoi.ru by Vetochka, who did the campaign.