I have noticed this as well. For example when you set the step size to 50 Hz if you tune quite quickly it seems that the step-size
Are you sure that this isn't intentional ? I see to recall Ian making a passing comment about variable rate tuning at some point.
P.S. I am really enjoying the ability to select the step size via touch screen in V2 of Ian's "Nextion" display code. This was a great addition.
I may have found another bug though. More on this later when I have had more of a chance to test it out.