Okay, discovered this one on my own.
I found that a Disguised role will never spoil. For instance, a Disguiser was killed with a Doc role.
When I used the role, the Doc role never spoiled. I could use it over and over and over to protect myself from attack.
I think it's because a Retributionist can't use Mafia roles, so they never spoil, BUT the disguised role is a Town role, so it'll be available anyway.
So the game never spoils the Mafia role, but the Disguised role is still accessible.
I hate to admit this one, because it's a neat exploit, but it should be changed so that the disguised role spoils like any other.
OR... leave it as is so there's a risk that the Mafia will give Retributionist an advantage if they screw up. (And admittedly, a Ret using the same person over and over is suspect as hell.)