After loading my game, the free standing walls got partly invisible:
I do not know what I did but after some time the walls got visible again.
You have the building vision mode activated 
The walls will show or hide depending on the direction in which you designed them. (I think this applies also for free standing walls, someone correct me if I’m wrong).
Phew, so many building bugs lately, it’s hard to keep the forum tidy enough… 
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