Cars don't count as blocking off an area.

[leon2356]

In unloaded zones, zombies can spawn in (or roam into) areas that are not blocked off. For example: A house has all doors and windows closed, zombies won't spawn inside it. House has a broken window? Then zombies can spawn inside it.

 

If you build a log wall around an area, then when you leave the zone and come back zombies won't be in the area due too there being no way to just walk in to it. But say you got a house and the door is broken, so you use a car to block the door. If you leave the zone and come back a zombie may spawn in the house due to the car not counting as a obstructing object like a player make wall or door would.

Edited November 13, 2019 by leon2356

[EnigmaGrey]

Not really a bug, since they were never meant to act as permanent, full-height walls in the first place. They're mobile; they don't line up with the tile grid; cars can be climbed over and under in reality if not in the game yet (with the exception of crawlers).