it seems that if you use Imbue Fireball (or whatever version) with a Blunderbuss then the first hit with a pellet will trigger the spell (Fireball). No additional Fireballs from additional pellets will follow. That's good. Multiple triggerings with multi-projectile weapons would be cheesy.
But the additional pellets get omitted completely. So you only deal the pierce damage of the first pellet and that's it. The rest gets canceled.
Expected behavior: first pellet triggers the spell, rest of the pellets hit as usual.