QNetworkAccessManager sends authorization header with blank user name

QNetworkAccessManager sometimes sends Digest authorization header even though neither QAuthenticator::setUser() nor QAuthenticator::setPassword() are ever called. I need to handle AuthenticationRequiredError myself and 90% of the time it works as documented but sometimes Qt resends the request with a Digest authorization header containing username="". According to the documentation, if neither setUser() nor setPassword() is called, finished() will be emitted with AuthenticationRequiredError. I am using Qt Creator 4.13.2 Based on Qt 5.15.1 (GCC 10.2.0, 64 bit).