Ah, yes. I've heard stories. I can imagine someone (or their agent) getting upset if they're hired for "Mobster #4" but then they're suddenly credited as "Mafia Don". The agent would have negotiated a better rate if the role was more central or prestigious. So sticking to the contracted name makes sense to avoid upset.
Reminds me of a story I remember hearing somewhere: One of the writers on The Simpsons told it. Apparently certain underhanded animated shows would occasionally try to get an actor for two shows for the price of one. The actors are just recording lines, so they don't necessarily know where they fit into an episode. Obviously their fee would be higher if they were going to be used in multiple episodes.
So an agent called up The Simpsons one time after their client recorded some lines and complained, "You've obviously tried to sneak multiple episodes out of my client! There's no way they could be in [crazy situation 1] AND [completely different situation 2] in a single episode!" But The Simpsons was so crazy that it wasn't unusual to go from an elaborate story about stealing a Gummy Bear, to a story about sexual harassment... in the same episode