HTTP/2 is enabled by default
In Qt 6 QNetworkAccessManager enables HTTP/2 protocol by default. Depending on the scheme ("https" or "http"), QNetworkAccessManager will use the Application Layer Protocol Negotiation TLS extension or "protocol upgrade" HTTP header to negotiate HTTP/2. If HTTP/2 cannot be negotiated, the access manager will fall back to using HTTP/1.1. If your application can only use HTTP/1.1, you have to disable HTTP/2 manually on a new request:

request.setAttribute(QNetworkRequest::Http2AllowedAttribute, false);