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It happens because of the implementation of permanent links (I think) in PropertyXhtmlString. If you don´t have any concerns about permanent links or any other special function (i.e. only using the property as a storage area), inherit from PropertyLongString instead.
I have a property which is a PropertyXhtmlString where the control inherits from PropertyDataControl.
When i use the SetValue method to save my xml strin geg. <root><string><![CDATA[mystring]]></string></root> the result is <root><string><![CDATA[<![CDATA[mystring]]>]]></string></root>. However, if i save <root><string>mystring</string></root> the saved value is the same as the submitted one.
Anyone have any ideas?