The RadExpander element gets underlined in the Visual Studio designer and you can see a NullReferenceException when mouse over the element. The issue occurs only when data bind the IsExpanded property of RadExpander.

There are no issues at runtime. Also, in the common scenario the designer doesn't break. In case the error breaks the designer, you can set the IsExpanded bindining in the code-behind, instead of XAML.

public MainWindow()
{
    InitializeComponent();
    this.radExpander.SetBinding(RadExpander.IsExpandedProperty, new Binding("IsExpanded"));
}

It fails to extract a template  using RightClick on RadExpander in Design view of VisualStudio => Edit Template => Edit a copy

Scheduled for:
The fix for this issue will be available with LIB (version 2018.3.1217) scheduled for publishing on Monday, 17th December 2018.
 

The expander toggle button arrow is pointing in the wrong direction. 

In Office2016/ Office2016Touch

Before expanding: "<"

After expanding: ">"

In other themes like Office2013, Green, VisualStudio2013, Windows8 etc.

Before expanding: ">"

After expanding: "<"

The fix is available in R1 2017 Official Release Version.

Implement a new property that allows you to apply a custom style on the expander ToggleButton

Will be available in Q1 2016 Release.

Fix the expander animation to take into account the Alignment properties. For example, we can't have expander which content is stretched vertically and the animation go top-down.