I cannot seem to create a chamfer on shape when starting below zero.
I have a circle that is pocketed to 10mm, then a second pocket that starts at that depth to through the stock. When I start a chamfer at the 10mm depth the choice for the depth is chamfer width but the preview throws error:
I played with your file, turning things off and on, as well as changing settings. I even tried switching the small pocket and chamfer to a blue layer.
Best I can tell, that inner chamfer references the final depth to the material surface instead if the lower step surface, which should indicate 13.46mm instead.
I am letting @JohnJohn know this looks like an Undocumented Feature.