QNetworkAccessManager emits signal authenticationRequired repeatedly when using NTLM SSPI authentication without proxy

Background information:

• Connecting to web server (IIS) directly, without proxy.
• The web server is set to accept any Windows user authenticating using
  NTLM (SSPI) when using SSL connection.
• Authentication works properly if I provide Windows username and password
  manually.

Behavior when authenticationRequired signal is emitted:

• Setting username and password correctly triggers NTLM authentication.
  The signal is emitted only once while loading a web page (documented
  behaviour).
• Not setting either username nor password causes 401 response from the
  web server. After this the signal is emitted again (documented behaviour).
• Setting username to empty string triggers NTLM SSPI authentication. The
  signal is emitted 4 times for each HTTPS GET request after which
  authentication succeeds. Password can be set to empty string or leave
  without setting, makes no difference.
• Setting username to empty string and password to some random string
  triggers NTLM SSPI authentication. The signal is emitted once for each
  HTTPS GET request after which authentication succeeds.
• Setting faulty username and empty password causes 401 response from the
  web server. After this the signal is emitted again (documented behaviour).

• Issue can be reproduced by Qt Demo Browser.