Probably not the first person to notice this, but I'm 99% sure that when an infected Bounty calls for a zombie that's rare or just uncommon (eg: scientist, bruiser, police) the likelihood of that zombie type spawning drop significantly below what would be on a normal mission. I've been keeping track over the last couple dozen bounties I've done and comparing bounties with the spawns you get when playing the same map in the same suburb when there isn't a bounty there's a measurable difference. It's not random.
Now I don't entirely mind this - bounties should be a challenge - but for some types of undead this makes bounties ridiculously hard, especially if you're playing without DMU. I feel like the devs may have overcompensated in making the undead bounties interesting. 4 missions to kill 5 police, which happened to me recently, is a little silly.