TLS instrumentation Test failed on Android Pie

I am testing now TLS data exchange between server and client in the context of instrumentation test.
The Handshake was successful, but data send by the server to the client, the client unwrap it but the buffer is full of zeros.
the message send by the client to the server is correctly decrypted.
As both client and server is on the same device and use the same api Boringssl instance is this the good way to test this?
If not is it a better way to do the test?

what seems bizarre to me is the fact that
on client side and on server
// Get a new instance of SSL Engine
mContext = SSLContext.getInstance(DEFAULT_PROTOCOL_NAME);
mEngine = mContext.createSSLEngine();
each class (server) then (client) has his own.

later one on client
**mEngine.setUseClientMode(true);**
on the server side
**mEngine.setUseClientMode(false);**

could I get a clarification on how to perform instrumentation test on Android for tls data transfert?