That's because Scarlet is so awesome. His greatness let him always flip the nasty debuffs back at the worst (best for him) ninja from your team.

He was born with it.

Maybe it's really just bad luck? Maybe it's Scarlet's magic?

I gotta test it on arena a bit- as Fire Main how that mirror targets enemies~~

On side note- I also got quite few complaints for Kushimaru always targetting best possible (for me) second ninja to use mystery on. Not quite true as it's not always so lucky. But moments of awesomeness happen.