I have seen different behavior between the two opponents. On the first scenario, the Bomber manages to throw the Big Bomb and then he is killed by the Cannon, on the second one, the Cannon manages to kill the Bomber before he throws the Big Bomb.

Now, this is not a bug. It is intentional. And the way to explain this is with the projectile that the Cannon shoots. It has a speed to connect to its intended target (in this case, the Bomber). The distance influences when it will arrive to its target and if the Bomber will manage to attack in time. In the first scenario, the distance between the two is slightly larger, enough to actually allow him to throw the Big Bomb , compared to the second one.

This mechanic works with any defense (or defensive troop) that throws projectiles. It does not happen with defenses with no projectiles such as Teslas and Crushers, neither against defensive Raged Barbarians.

So that means that when using Bombers (or any ranged troop for that matter), it is better to place it at exactly where the troop's maximum distance is. This could get you an extra attack.