I would either expect the slider to be slideable to the far right end value regardless of the TickStep interval by default. OR there should be a property that allows the slider to be dragged to the end ignoring the TickStep when SnapMode property is set to SnapToTicks
Here is the setup:
    <VerticalStackLayout Spacing="25"
                         Padding="30,0"
                         VerticalOptions="Center">
        <telerik:RadSlider Minimum="1" x:Name="slider"
                           Maximum="15" 
                           TicksPlacement="End"
                           SnapMode="SnapToTicks"
                           LabelsPlacement="End"
                           TickStep="3" />
    </VerticalStackLayout>and the result, I cannot drag the thumb to maximum value which is 15.