As can be seen in the attached repro mission, AI units are unable to move through spaces between objects smaller than 3x3 meters or so.

Physics objects like vehicles or furniture further confuse them as the AI has an especially hard time finding their way around them.

AI also seems to be able to sometimes walk through objects, especially player controlled cars if they are unable to find a way around.

See pictures for the most miserably stuck units. {F22174} {F22175} {F22176}