This happens because in the current version of Telerik, the IsTopmost of the ToolWindow that host the floating pane is set to True. That was needed for a related new functionality, but it brings a major behavioral change in the RadDocking control.

To work this around, you can subscribe to the ToolWindowCreated event of RadDocking and set the IsTopmost property of the creating ToolWindow to False.

private void RadDocking_ToolWindowCreated(object sender, Telerik.Windows.Controls.Docking.ElementCreatedEventArgs e)
{
    var window = (ToolWindow)e.CreatedElement;
    window.IsTopmost = false;
}

When a RadSplitContainer with Orientation="Vertical" contains RadPaneGroup instances, resizing them causes a flicker when the ShowResizePreview property is set to False. This happens with the VisualStudio2019 theme.

For the time being, a possible workaround is to create a global Style with TargetType="RadPaneGroup" and set the Margin property to "0":

<Style TargetType="telerik:RadPaneGroup" BasedOn="{StaticResource RadPaneGroupStyle}">
    <Setter Property="Margin" Value="0"/>
</Style>

The DataContext of RadPane's content is lost when showing its preview in the DockingNavigator.

As a workaround, you could set the DataContext of the content element could be set explicitly.

The RadOutlookBar control takes a significant amount of time to load initially when it does not have a fixed width and its IsMinimized property is explicitly set to True.

A possible workaround, for the time being, is to set a fixed Width for the control and change its value in the Minimized and Restored events.

Currently, the RadPaneGroup has one Grid layout with Margin="5" present in its control template, which causes the Content of each child pane to be indented according to the Margin property's value.

To work this around, extract the control template, with x:Key="RadPaneGroupDocumentHostControlTemplate", and modify the Grid layout, which has its Margin property set to 5. After that, create a new style with TargetType="RadPaneGroup" and set its DocumentHostTemplate property, to the modified one.