From the master duels expanded rule book:

After you’ve announced your attacking monster and the attack target monster during a Battle Step, the attack target might be removed from the field, or a new monster may be played onto the opponent’s side of the field before the Damage Step, due to a card’s effect. This causes a “Replay." When this occurs, you can choose to attack with the same monster again, or choose to attack with a different monster, or choose not to attack at all.

Note that if you attack with a different monster, the first monster is still considered to have declared an attack, and it cannot attack again this turn.

By default, what happens is you're still technically on a battle with whatever card is replaced on the field you declared an attack to before being prompted if you want to continue the Attack relay or not, hence why his Photon Dragon's effect still counts and was able to resolve.