It appears that QNetworkAccessManager can keep receiving queued signals even after it has cleared its reference to QNetworkSession and disconnected the slots. That can mess up networkAccessible state. Particularly, QNetworkAccessManager can get stuck in NotAccessible state and refuse to submit new network requests until the next online state change signal pulls it out of this state.

There still may be a possibility that a signal from an old QNetworkSession could arrive when QNetworkAccessManager has already switched to another QNetworkSession, but this scenario is probably broken by design, since for whatever reason QNetworkAccessManager is only tracking one QNetworkSession. In any case, it's far less likely than the scenario which I was trying to fix.