The overload of QSslSocket::setLocalCertificate taking a file path will succeed silently in all cases (also applicable to setting a null certificate). Having at least one of the following three things as error reporting is not unreasonable to expect:
1) The method returns a success status (bool, probably can be added for Qt6).
2) The method emits a debug warning (qWarning).
3) The socket emits an SSL error.

The offending code is here:
qtbase/src/network/ssl/qsslsocket.cpp