The problem is that even if the "packagesToScan" works...which it does the supportInternal method needs to check the current JAXBContext to see if that class is actually loaded. I checked and you cannot do this with JAXBContext so a better solution might be to add the class names to "classesToBeBound" as they are scanned in from the package scanner. After that then in the "supportsInternal" method an additional check can be made to see if the packages.
If you need me to attach code for the fix I would be more than happy to. Please just let me know as I am here to help. Also I didn't check the latest spring-oxm so I am not sure if this is no longer an issue but I couldn't find information on this in jira.
#13790 Jaxb2Marshaller::supports() ignores classes found by packages scanning ("duplicates")