You post makes no sense at all. There's no bug with Black Mantle. Black Mantle lowers their accuracy by 45% for the next spell cast, increasing their chance of fizzle. That's exactly what it's supposed to do. Black Mantle has no effect on the strength of an attack, it only affects whether or not they fizzle. Weakness is the spell that affects attack damage by -25%
It's not a bug. Black Mantle decreases the accuracy by 45% to ALL spells, shields included. Remember, it just decreases the accuracy. It doesn't dispel it. So there will always be a chance that they can cast the spell that they were about to cast. So once a spell is cast, regardless whether or not the spell fizzles, the black mantle will go away. I hope I helped.