Yeah I think a mod needs to pin something like this:
At around level 30, more and more zombies get stuck outside the building, some still come running in and others vanish into thin air when they try and attack you. This happens to everyone and it hasnt been fixed or anything like that.
As a player you have no control over when it happens or how it happens, you just have to be good enough to get that far. There are rumours that the zombies come in at level 40, I have tested this and got to 45 and it was still in affect. Other rumours say that they come in at 60 again, but I cant be asked to play for that long and no-one should.