I highly doubt it's because of holes in the mesh. You could have a road in the sky and still work, afaik.
Maybe it's related to the way the borderlines are detected, as Rob says?
Make a screenshot when you have the AI app active, it should show the borderlines in red.
A simple way to get rid of "noise", is to edit the surfaces.ini and define all keys (including pits) as invalid (i.e. IS_VALID_TRACK=0, iirc), except for the road key itself. Then try again.
Maybe it's related to the way the borderlines are detected, as Rob says?
Make a screenshot when you have the AI app active, it should show the borderlines in red.
A simple way to get rid of "noise", is to edit the surfaces.ini and define all keys (including pits) as invalid (i.e. IS_VALID_TRACK=0, iirc), except for the road key itself. Then try again.