I've wrote the comment for old issue.
Comments: I've updated Ajax Control Toolkit. And CascadingDropDown always set default value from web method (after any postback event). If I change the value and push the button with Autopostback="true" CascadingDropDown set default value too. When I return the old version, all works properly.
Comments: I've updated Ajax Control Toolkit. And CascadingDropDown always set default value from web method (after any postback event). If I change the value and push the button with Autopostback="true" CascadingDropDown set default value too. When I return the old version, all works properly.