With the support for TLS 1.3 upcoming, if you enable the #define QT_DECRYPT_SSL_TRAFFIC and negotiate on TLS 1.3 the master secret will be all zeros in the qt-ssl-keys file.

This is due to TLS 1.3 only having the session ticket information available after the handshake has already completed. https://www.openssl.org/docs/man1.1.1/man3/SSL_get_session.html:

The ssl session contains all information required to re-establish the connection without a full handshake for SSL versions up to and including TLSv1.2. In TLSv1.3 the same is true, but sessions are established after the main handshake has occurred.

Hence why the QT_DECRYPT_SSL_TRAFFIC code in continueHandshake() within qsslsocket_openssl11.cpp does not work with TLS 1.3 connections.