I tried to reproduce the problem you describe, but to no avail - please check the attached screenshot. Below is the code of my test page:
|<telerik:RadSlider ID="RadSlider2" runat="server"></telerik:RadSlider>
|<telerik:RadSlider ID="RadSlider1" runat="server" Orientation="Vertical"></telerik:RadSlider>
Could you please prepare and send us a simple running project, demonstrating the problem so that we can research what is causing it? It would be best if you could send us a video showing the behavior you get as well.
All the best,
the Telerik team
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