I don’t use the Polygon Command very much but I noticed that, if you set the mode to Circumscribed, Rhino resets it back to Inscribed each time so that there is no persistence in the setting.
I am not seeing that here in a current Rhino 7 version.
I can tell you the difference.
If you do the polygon command by clicking the button on the panel it resets (what I did). If you do it by the menu or the command line it remembers.