How can one modify a response to RTSP's SET_PARAMETER

Dear community,

I am trying to use SET_PARAMETER to send some information from the 
server to the client. On the server side, I subclassed GstRTSPServer and 
GstRTSPClient. My RTSPServer responds to a create_client call and 
returns my RTSPClient. My RTSPClient responds to params_set, analyzes 
the content and sets a response message. Since I don't know another way 
to send a response with my message, I decided to send it in the "reason":

         gst_rtsp_message_init_response(
             ctx->response
           , (GstRTSPStatusCode)code
           , "arbitrary text"
           , ctx->request
         );

The log output indicates that the response message was correctly created:

RTSP request message 0x793e58
  request line:
    method: 'SET_PARAMETER'
    uri:    'rtsp://127.0.0.1:18554/distance/'
    version: '1.0'
  headers:
    key: 'CSeq', value: '5'
    key: 'Content-Type', value: 'text/parameters'
    key: 'Session', value: 'stXFg.MwlZw2_EQ.'
    key: 'Date', value: 'Fri, 08 Sep 2023 20:26:14 GMT'
    key: 'Content-Length', value: '28'
RTSP response message 0x7fffe77fd890
  status line:
    code:   '200'
    reason: 'arbitrary text'
    version: '1.0'
  headers:
    key: 'CSeq', value: '5'
    key: 'Server', value: 'GStreamer RTSP server'
    key: 'Session', value: 'stXFg.MwlZw2_EQ.'

On the client side, I use g_signal_emit_by_name on RTSPSrc to emit 
"set-parameter". It returns a promise with the following structure:

     set-parameter-reply, rtsp-result=(int)0, rtsp-code=(int)200, 
rtsp-reason=(string)OK;

You can see that my message has disappeared. How do I send a message in 
the response?

Best.
Andrey.