Standard Plain JAX-RS services breaks with JAXBException

The REST services set up through Essentials, see "Custom Plain JAX-RS Services" at https://xmdocumentation.bloomreach.com/library/setup/development-tools.html#rest-services-setup, produces a JAXBException

Reproduction

• run archetype 14.7.9 or 15.1.1
• build, run, install Simple Content feature with Essentials
• use Essentials REST services tool, select custom, Content Document and URL "rest"
• rebuild, rerun, visit http://localhost:8080/site/rest/ContentDocument/

Result on screen:

javax.xml.bind.JAXBException: class org.example.beans.ContentDocument$Contentdocument$6YJygevH nor any of its super class is known to this context.

and a long stack trace in the back-end coming from org.hippoecm.hst.jaxrs.cxf.CXFJaxrsService.invoke.

Note it is not related to dynamic beans, when setting allowModifications=false in the content bean, the same happens with org.example.beans.ContentDocument.