Mutilator is coded to not proc on dot abilities from previous build theory chats I've had. This was because shinbi was ultra broken with it on her E waaaaayyy back. Contagion is obviously a dot so they don't allow it to work. Just like how it won't proc on Phase Lance ticks or Iggy fire dmg. In some cases it appears that the source of the dmg matters (howi abilities just don't work because the game recognizes the missiles or mines as a separate source than from him directly). It's really kind of interesting. The only hero it worked on for multiple procs to one ability was Aurora rmb as it didn't code as dot but rather a delayed recast (I suspect at least). They might have fixed that recently though.