Reply To: Sub-Div or not Sub-Div
I think you have to recalculate this face here Do you check the Face Orientation? You can activate this option here (see image).
If faces are red, they must be recalculated. To do this you have to go to Edit Mode -> Face Mode -> select all faces or only those that are red and then ALT + N.
If it is only one face, flip option is sufficient, if several are affected, use the recalculate functions.
