This may be old, don't know if you still looking for a solution to this. This is indeed a feature of how the normals are facing when doing an inset with depth. The result you're looking for can be achieved by doing an inset then performing an extrusion.
3 Comments
You may want to report this a bug instead.
@Emil Assarsson: I sent the report. I was told that this is not a bug developer.blender.org/T60707
This may be old, don't know if you still looking for a solution to this. This is indeed a feature of how the normals are facing when doing an inset with depth. The result you're looking for can be achieved by doing an inset then performing an extrusion.