So there's some weirdness in how FG handles text changes. After you change the text in that field to "Weapon HP" can you try tabbing to the next text field, then trying it? It could be storing the old roll (which uses stun) until you deselect the field. If that still fails, can you try rolling from the same text but directly from the NPC sheet?
If that doesn't work, try replacing the existing extension with the version attached here. Not sure if it'll make a difference, but just checking stuff.
(still looking into the effort issue)