When the Final Shout enchantment activates at 25% health, it uses all your current artifacts. If the artifacts were already active, they cannot be used again unless unequipped and reequipped. They come off cooldown, but stay greyed out in the bar and selecting them does nothing.

I've tested this many times with the missile artifacts (Fireworks Arrow, Flaming Quiver, and Torment Quiver), they are all affected.