We thought you are using RadAutoCompleteBox without RadExpander. We saw your project and this is the same bug described in the previous thread. Unfortunately, we are not aware of any clean solutions. On a side note, the type of the Input control does not matter significantly - this is reproducible with TextBox, TextInput, AutoCompleteBox in the Expander's Header.
However, it is strange to use AutoComplete/RadAutoCompleteBox in the header of Expander. Actually, our RadAutoCompleteBox expands if it finds an item containing the string you type in. It is unusual to have expanding control nested in expanding control.
the Telerik team