I confirm that this is an issue with our RadTrackBar element and I have logged it in our FeedBack Portal. You can track the item for status changes and add your vote for it here.
You could workaround the issue by setting the StretchHorizontally property of your RadTrackBar element to false. You would also need to apply a margin and adjust its position in the DropDownButton element. Please see my code below:
public partial class Form1 : Form
RadTrackBarElement trackBar = new RadTrackBarElement();
trackBar.StretchHorizontally = false;
RadMenuItem menuItemTrackBar = new RadMenuItem();
menuItemTrackBar.Margin = new Padding(0, 0, -25, 0);