[Rendar]

Putting a spell scroll onto a weapon that isn't Spelledged and/or a tome will automatically use that weapon as the casting implement for said spell scroll. Even if you have another weapon that is ACTUALLY a valid casting tool.

See: Kusarigami mainhand with Raven Spellscroll and Reaper, and MMA off-hand with Fist Tattoo. It will always cast raven with the Kusarigami, regardless of Ambidex, etc.

[Neus]

Not a bug. This is how spells/skills conferred by weapon parts work.