workaround:
add the following visual state to the DocumentHostTemplate of the RadPaneGroup:

<VisualStateManager.VisualStateGroups>
    <VisualStateGroup x:Name="DropDownDisplayStates">
        <VisualState x:Name="DropDownButtonCollapsed" />
        <VisualState x:Name="DropDownButtonVisible">
            <Storyboard>
                <ObjectAnimationUsingKeyFrames Storyboard.TargetName="DropDownButtonElement" Storyboard.TargetProperty="Visibility">
                    <DiscreteObjectKeyFrame KeyTime="0">
                        <DiscreteObjectKeyFrame.Value>
                            <Visibility>Visible</Visibility>
                        </DiscreteObjectKeyFrame.Value>
                    </DiscreteObjectKeyFrame>
                </ObjectAnimationUsingKeyFrames>
            </Storyboard>
        </VisualState>
    </VisualStateGroup>
</VisualStateManager.VisualStateGroups>

and remove the trigger targeting the drop down button.