I don't think this is a bug... I think this is the way the game is designed. You must stay in the bombsite until all of the bombs are out of the bomb bay or
they will cease to drop as you have already experienced.
Think of it this way...you are either the Bombardier, or a Gunner...you cannot be both of them at once; so once you leave the Bombardier's position and take up Gunner duties within the aircraft, the Bombardier quits working.
Just a thought...