xmlrpc.client.Fault when calling an xmlrpc method from rpyc

I'm trying to build a system with two processes: an xmlrpc server with a "ping" method and a QCoreApplication middleware. The middleware has an xmlrpc client which connects to the xmlrpc server. The middleware also acts as an RPyC server. I want to connect to the middleware via RPyC and call the "ping" method on the server.

# xmlrpc_server.py
from xmlrpc.server import SimpleXMLRPCServer


class XMLRPCServer(SimpleXMLRPCServer):

    def __init__(self, addr=('localhost', 4444)):
        super(XMLRPCServer, self).__init__(addr)
        self.register_function(self.ping)

    def ping(self):
        return 'pong'


if __name__ == '__main__':
    s = XMLRPCServer()
    s.serve_forever()

# middleware.py
import sys
import xmlrpc.client
import rpyc
import rpyc.utils.server
from PyQt5 import Qt


class MiddleWare(Qt.QCoreApplication):
    def __init__(self, *args, **kwargs):
        super(MiddleWare, self).__init__(*args, **kwargs)
        self.xmlrpc_client = xmlrpc.client.ServerProxy('http://localhost:4444')
        self.rpyc_server = RPyCServer(port=2222)
        self.rpyc_server.start()

    def ping(self):
        return self.xmlrpc_client.ping()


class RPyCService(rpyc.Service):

    def on_connect(self):
        self.exposed_application = Qt.QCoreApplication.instance()


class RPyCServer(Qt.QThread):

    def __init__(self, port=None):
        super(RPyCServer, self).__init__()
        self._server = rpyc.utils.server.ThreadedServer(
                RPyCService,
                port=port,
                protocol_config={
                    'allow_all_attrs': True,
                    'allow_public_attrs': True,
                },
        )
        self.run = self._server.start


if __name__ == '__main__':
    mw = MiddleWare(sys.argv)
    sys.exit(mw.exec_())

$ python3 xmlrpc_server.py &
[1] 5785
$ python3 middleware.py &
[2] 5986
$ python3
Python 3.6.7 (default, Oct 22 2018, 11:32:17) 
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import rpyc
>>> conn = rpyc.connect('localhost', 2222, config={'allow_all_attrs': True})
>>> conn.root.application.ping()
'pong'
>>> conn.root.application.xmlrpc_client.ping()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3/dist-packages/rpyc/core/netref.py", line 199, in __call__
    return syncreq(_self, consts.HANDLE_CALL, args, kwargs)
  File "/usr/lib/python3/dist-packages/rpyc/core/netref.py", line 72, in syncreq
    return conn.sync_request(handler, oid, *args)
  File "/usr/lib/python3/dist-packages/rpyc/core/protocol.py", line 523, in sync_request
    raise obj
rpyc.core.vinegar/xmlrpc.client.Fault: 

========= Remote Traceback (1) =========
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/rpyc/core/protocol.py", line 347, in _dispatch_request
    res = self._HANDLERS[handler](self, *args)
  File "/usr/lib/python3/dist-packages/rpyc/core/protocol.py", line 624, in _handle_call
    return self._local_objects[oid](*args, **dict(kwargs))
  File "/usr/lib/python3.6/xmlrpc/client.py", line 1112, in __call__
    return self.__send(self.__name, args)
  File "/usr/lib/python3.6/xmlrpc/client.py", line 1452, in __request
    verbose=self.__verbose
  File "/usr/lib/python3.6/xmlrpc/client.py", line 1154, in request
    return self.single_request(host, handler, request_body, verbose)
  File "/usr/lib/python3.6/xmlrpc/client.py", line 1170, in single_request
    return self.parse_response(resp)
  File "/usr/lib/python3.6/xmlrpc/client.py", line 1342, in parse_response
    return u.close()
  File "/usr/lib/python3.6/xmlrpc/client.py", line 656, in close
    raise Fault(**self._stack[0])
xmlrpc.client.Fault: <Fault 1: '<class \'Exception\'>:method "exposed_ping" is not supported'>

>>> 

I expected to get the value 'pong' back from the call, both when calling conn.root.application.ping() and directly conn.root.application.xmlrpc_client.ping(), instead in the second case an error was shown. Why?

I was able to get the desired result by editing the [protocol_]config, adding

'exposed_prefix': ''

# middleware.py
#   ...

class RPyCServer(Qt.QThread):

    def __init__(self, port=None):
        super(RPyCServer, self).__init__()
        self._server = rpyc.utils.server.ThreadedServer(
                RPyCService,
                port=port,
                protocol_config={
                    'allow_all_attrs': True,
                    'exposed_prefix': '',
                },
        )
        self.run = self._server.start

#   ...

$ python3
Python 3.6.7 (default, Oct 22 2018, 11:32:17) 
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import rpyc
>>> conn = rpyc.connect('localhost', 2222, config={'allow_all_attrs': True, 'exposed_prefix': ''})
>>> conn.root.application.ping()
'pong'
>>> conn.root.application.xmlrpc_client.ping()
'pong'
>>>