If you use the 3rd part of the Shield Master Feat;
If you are subjected to an effect that allows you to make a Dexterity saving throw to take only half damage, you can use your reaction to take no damage if you succeed on the saving throw, interposing your shield between yourself and the source of the effect.
If another Fireball is dropped in the same spot, during the same round, but later in the Initiative Order, does the shield still help you? Or if a wagon full of explosives goes off, the DM wants it to work for only one bomb in the crate, not all of them. Many Reactions, once activated, keep working for the rest of the round. Since the Shield is still in the same protective place and the source is still the same, does it still provide the same protection?
Let me again be very clear. Same round. Same locations for the character, shield and the source of the effect. Does Shield Master continue working?