I'm also having the same issue. For me it only happens on an Alpha 18 dedicated server that I run. It happens in both RWG or the preset map. Size makes no difference. Zombies will try and attack me if I get close, but they do not move.
So far, this has not happened in single player on the same computer, but then I'm running the standard local client rather than the dedicated server.
I'll do some troubleshooting over the weekend and report back including logs.
Have any of you guys above found any solutions yet?