41.71
Singleplayer
Reproduction steps:
Build wall frame on the edge of the flat roof at Riverside Factory Warehouse
Try to add wall (I tried level 2 and later level 3 wall). The green wall appears to confirm it can be built, but no action occurs and no wall is built.
If the wall frame is facing SW or SE, you can only build the wall by getting around to the other side. Adding a wall works fine for the NE/NW facing wall frames.
There is a work-around - build wooden floors on the other side of the wall frame, then you can build the wooden wall as normal, from either side of it.
I have added a photo showing this bug.