I was wondering why does using Face fillet creates a spline. I just noticed this while I was creating a part and my VAR confirmed that it is intentional.
IMHO, it should always be a radius. The intent of using the Fillet command is to add radiuses on edges. My VAR advised me to put the radiuses in a sketch but that is out of the question. Most people know fillets shouldn't be in sketches...
Part attached
That's a full round fillet not a face fillet.
So, given the above geometry, you can see why selecting the fillet doesn't display a radius in the status bar. Your model is a degenerate case where the radius is constant from one side face to the other, but it doesn't have to be.