Enum Multipart parameter

I'm currently struggling with the following snippet which used to work with TomEE 8.x but fails on 9.1.2:

@POST
@Path("/documents/{type}/by-id/{id}")
@Consumes(MediaType.MULTIPART_FORM_DATA)
@Produces(MediaType.APPLICATION_JSON)
public Response postWithId(@PathParam("type") String type, @PathParam("id") int id,
                                        @Multipart(value = "file") Attachment attachment,
                                        @Multipart(value = "category") DocumentCategory category,
                                        @Multipart(value = "description", required = false) String description) {
      // ...
}

DocumentCategory is an Enum. A POST request with an example file and the category set to "GENERIC" invokes the function with the category parameter set to "null".

Doing things by hand works just fine, though:

@POST
@Path("/documents/{type}/by-id/{id}")
@Consumes(MediaType.MULTIPART_FORM_DATA)
@Produces(MediaType.APPLICATION_JSON)
public Response postWithId(@PathParam("type") String type, @PathParam("id") int id,
                                        @Multipart(value = "file") Attachment attachment,
                                        @Multipart(value = "category") String category,
                                        @Multipart(value = "description", required = false) String description) {
      DocumentCategory cat = DocumentCategory.valueOf(cat);
      // ...
}

Am I missing something here?