The RadExpander is using internally the built-in ToggleButton control and the IsExpanded property is bound to the ToggleButton.IsChecked. But unfortunately the ToggleButton is changing IsChecked on the ClickEvent which is not raised when you Touch, but only on real mouse click. The best solution to the problem is if Microsoft fix the ToggleButton, but while this happens, we should find another solution, so I'll create work item for it.
Sorry for any inconvenience.
All the best,
the Telerik team
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