I've made a test using the provided configuration of the RadComboBox and indeed there is an issue with the TextChange event, which as pointed in your post, does not fire when the first item is unchecked. Further testing this issue, I have noticed that this behavior is reproduced only when the RadComboBoxItem has value set as well as text. In your particular scenario one and the same information is being bound to the Text and the Value properties of the RadComboBoxItem, so please try not to bind the Value property and use only the text and let me know whether the issue persists?
As a token of gratitude for this bug report your Telerik points are updated.
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