Sometimes there are no zombies. When there are they don't show on the radar. Is this temporary, will it be fixed or does no one care?
You can / should try submitting a ticket with Rockstar Games. Check here:
support.rockstargames.com/.../new | Whether or not they get around to fixing it is hard to estimate - however, I did run into this bug myself a few times.
Never had this happen to me actually. But RDR multiplayer is filled with bugs.