What are the dimensions of the rectangle? Is it possibly so small that only one increment is feasible?
If the width and/or height of the rectangle is equal to or smaller than the Preferences > Corner Radius setting, there is only one step.
Are you using a Wacom tablet? There's an issue with Wacom drivers which cause the corners to RAPIDLY change which at times may appear like one step unless you very quickly TAP the arrow key.
This was fixed for a version or two, then crept back in.
I don't understand those last two posts, Paolo. Does that mean you do indeed use a Wacom? If so, it's up to Wacom to fix it and I would not hold my breath. It's been an issue for litterally a decade or more.