i got the issue when i click on seconds the seconds highlights but spinner only updates the hour,
the same goes for minutes, the spin works only for hours.

When i used the pulldown also only 01:00:00 appears (with the following full hours) which is ok

my xaml entry is,

<xctk:TimePicker Grid.Column="1" x:Name="startTimePicker" Format="LongTime" FormatString="HH:mm:ss" HorizontalAlignment="Left" Margin="5,70,0,0" VerticalAlignment="Top" Width="100"/>

i am using Version 4.7.25104.5739