In the gym room, the second floor in if you follow the path from outside/mission start, two zombies are aggroed and appear on the radar, but as soon as their aggro sound starts playing the sound fades away. When leaving the room the red dots disappear. When in the room, the zombies are heard and (re)aggroed, but are nowhere to be found. This makes it impossible to finish a Clear mission.
PC, clean wipe, new game, second time I tried the mission with the same weird bug.