This behavior is reproducible when we have applied Material theme and we have disabled the selection and drag of a RadTreeViewItem.
A possible workaround is to set the MaterialAssist.CheckedBrush attached property to Null of the RadTreeViewItem which you don't want to select and drag.
xmlns:material="clr-namespace:Telerik.Windows.Controls.MaterialControls;assembly=Telerik.Windows.Controls"
<telerik:RadTreeViewItem Header="RootItem" material:MaterialAssist.CheckedBrush="{x:Null}" />
<telerik:RadTreeViewItem Header="RootItem 1" IsSelectable="False"> <telerik:RadTreeViewItem Header="ChildItem 1" /> <telerik:RadTreeViewItem Header="ChildItem 2"/> <telerik:RadTreeViewItem Header="ChildItem 3"/></telerik:RadTreeViewItem>