There's a video on YouTube by a guy who said that after killing all the zombies, you get a syringe to use. I killed all the zombies in the mini-event and got nothing. Only the locked room opened with loot.
When you play the Metro Royale mode and do it, they do give you a syringe, but the mission progress doesn't count if you are doing anything in Metro Royale mode. It doesn't even count the zombies you kill.
So, is this a bug? I can't complete these missions.