Hi,

The fix for the issue will be available in next LIB version (2018.2.604). It will also be included in our next official version – R2 2018 SP1, scheduled for the end of June.

Best regards,
Ralitsa Kumanova

We discussed the priority of this issue once again and we also scheduled a bug fix for the incoming internal build next week.

On a side note , another workaround could be to wrap the UIElement defined as child of RadPanelBarItems in other RadPanelBarItems' Headers. This way the UIElement will have proper Logical Parent - RadPanelBarItem and its DataContext will be inherited properly.

Here is the example:

<RadPanelBar>
    <RadPanelBarItem Header="Root Item">
        <Button Content="Simple Button" />

In this case the Button has logical parent - RadPanelBarItem but is automatically wrapped internally in other new RadPanelBarItem (which is ItemsControl) which is his visual parent but not logical one.

To proper inherit DataContext, if you use 2018 R2 version, you can use the following code:

<RadPanelBar>
    <RadPanelBarItem Header="Root Item">
         <RadPanelBarItem
              <RadPanelBarItem.Header>
                  <Button Content="Simple Button" />

which will produce the same visual result.

The issue is not scheduled for the incoming R2 2018 SP Release in June. 
We encourage you to set the DataContext of the problematic element directly.