RadFormDecorator will not decorate a RadComboBox because it is a control that is already styled. RadFormDecorator works on basic form elements like checkboxes, inputs, headings that are not already decorated by the developer (i.e. have no class set).
Regarding your original issue - we have seen in in an older version, as my colleague Danail already said, yet we have not observed in the current one. Have you tried setting the EnableRoundedCorners property to false?. Can you reproduce the behavior you experience on our online demos
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