Why do grey faces appear on inset faces after using the knife tool? When I use the knife tool with angle constrained on an inset face it leads to these weird grey patches appearing on the face and I don't know what they are.

Is there any way to use the knife tool on an inset face without this happening?

Before simple knife cut:



After simple knife cut (highlighted):
It looks like the inset face has some bad geometry. The right-most edge might not actually be a boundary for the face. Try deleting the face &amp; remaking it.

If that doesn't work, breaking up the N-gon into quads might help.