public class Params {
    @PathParam("p1")
    private String p1;
    public String getP1() {
        return p1;
    }
    public void setP1(String p1) {
        this.p1 = p1;
    }
}
@Path("/a")
public interface Proxy {
    @GET
    @Path("a/{p1}")
    String getAll(@BeanParam Params beanParam);
}
@Path("/a")
public class Resource {
    @GET
    @Path("a/{p1}")
    public String getAll(@BeanParam Params beanParam) {
        return beanParam.getP1() + "_" + beanParam.getP1();
    }
}

public class App {
    private static ResteasyClient client = new ResteasyClientBuilder().build();
    public static void main(String[] args) throws Exception {
        Params params = new Params();
        params.setP1("test");

        // normal client usage works as expected
        Response r = client.target("http://localhost:8080/jaxrs-wf/a/a/test").request().get();
        System.out.println(r.readEntity(String.class));

        // proxy client usage works as expected
        Proxy apiService = client.target("http://localhost:8080/jaxrs-wf/").proxy(Proxy.class);
        System.out.println(apiService.getAll(params));

        // MP rest client fails
        RestClientBuilder builder = RestClientBuilder.newBuilder();
        Proxy restClient = builder.baseUrl(new URL("http://localhost:8080/jaxrs-wf/")).build(Proxy.class);
        System.out.println(restClient.getAll(params));
    }
}