I have just downloaded the file that I shared with you, the .3dm file, and I can replicate your results (actually not entirely, some of the faces are aligned with neighbor faces like shown before). But if I create a cube from scratch, apply a chamfer, and then try to use the Inset function, I still get the funky results I have posted here. I'm totally puzzled now. LOL.

Looks like the 3dm file format is fixing whatever issue is going on.