[Autumn]

Personally, I wouldn't want it being capable of doing the full damage. If this were a bug, I'd hope there was a reduction to the damage of some sort. Would be very annoying to come across in battle if it wasted your momentum to then pump a massive attack back at you, since you can't use projectiles on someone without reflecting it first, making it not really avoidable.

If it were to retain the distance already traveled at the very least it would not be too hard to deal with either, allowing you to remain at a safe distance from your own projectile so long as you're observant. (as its revealed by a status.)

For the case of big ultra death moves, having those without a smaller projectile to proc mirrors first is sort of a risky way to play around it, and more so what spirit mirror's strengths are as an item slot comparatively to other powerful accessories, I don't think a damage reduction would be necessary.