I would suspect one or two things.
1. is it in a complex profile wall as Minh has asked?
If so the opening reference lines in the complex profile may not have been set properly.
2. Is is a custom saved door/window object.
If so it may not have been created properly with the slabs set at zero height.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10