Opponent was able to cast Imp's Mischief targeting my Decisive Denial. I chose the first mode of Decisive Denial and targeted my creature and their creature. Opponent then cast Imp's Mischief targeting the spell, and didn't or wasn't able to change the target. If I've understood the rules correctly they shouldn't be able to cast Mischief targeting Denial to begin with, since it has multiple targets.

Steps to reproduce: Cast Decisive Denial choosing the first mode. Cast Imp's Mischief.
Playing on PC.