I assumed it was actually working as intended, given it has the same 'twirling arrows' mechanic indicator as that mechanic in trials/raids, and those usually cannot be interrupted with Stun. Also it makes an odd kind of sense, it's a machine so bashing it wouldn't cause it to suddennly terminate that operation, it would momentarily pause it, and then resume it. So it makes sense Stun doesn't fully cancel the mechanic.

It's easy enough to deal with anyway.