Unable to inset N-gon face I am trying to inset N-gon face but failing.
Create cube. Delete top and bottom faces.

Add separation with Ctrl + R so that it is not in the middle of a face:

Fill top hole with F. It will be a 5-gon. Select it and start inset

Make it smaller than the distance between center and additional separation. Dithering begin.
Is n-gon prohibited situation? How to fix out of it?
Instead of filling the face and insetting, you can extrude the edge with E and scale it down by pressing S afterward.