Document?
Sources?
Well, the real stuka had the armament selection panel in the right cockpit.
One lever for the fuselage bomb, four for the wing bombs.
You could drop every bomb singly or in any sequence or combination you wanted.
We can't do that in Il-2, but standard procedure (according to a number of books I read including "Stuka Aces" and the memoirs of I./StG 1 commander Hellmuth Mahlke) was that the 250 kilo bomb was used in the initial dive and the 4 50 kg bombs were often retained to engaged targets of opportunity during strafing runs.
So it makes a lot more sense to drop the heavy bomb first. Dive, drop heavy bomb, pull out, use rest of the bombs in secondary attacks.
Why should you drop the small bombs first? You can't climb back to any decent altitude over the target with the heavy bomb anyway.
For the AI it makes no difference as it uses all at once anyway.
The Ju 88 has it set this way.
And bomb drop was not automatical, the signal horn was just a reminder to drop.
It used to be like that once in Il-2, if I remember right.