This behavior is observed in MS TextBox and RadTextBox. In RadTextBox is observed because internally the control host MS TextBox. So this behavior derives from MS TextBox. In a few words, if the RadSpellChecker control does not have a suggestion for the wrong spell word it should give the user an option to add it to a custom dictionary. Instead, the default MS ContextMenu appears.

A possible workaround is to remove the default MS ContextMenu:

Me.TextBox1.ContextMenu = New ContextMenu
Me.RadTextBox1.TextBoxElement.TextBoxItem.TextBoxControl.ContextMenu = New ContextMenu