QNetworkAccessManager::proxyAuthenticationRequired fired multiple times

1. Use a proxy which requires authentication. Either specify it in global system settings, or in app using Qt API.
2. Connect to QNetworkAccessManager::proxyAuthenticationRequired and provide user name and password there.
3. Bug: in some cases Qt fires this signal several times, despite the fact that credentials are always correct.

I've made some investigation and found that this happens when you set custom QNetworkRequest::UserAgentHeader and trying to access some specific URLs.

E.g. some cases:

1. Just: QNetworkRequest request(QUrl("https://rutracker.org/forum/dl.php?t=4005246"));

Problem is NOT happening. Log is showing:
onProxyAuthenticationRequired

2. QNetworkRequest request(QUrl("https://rutracker.org/forum/dl.php?t=4005246"));
request.setHeader(QNetworkRequest::UserAgentHeader, "FDM/6.0.0.0");

Problem IS happening. Log is showing:
onProxyAuthenticationRequired
onProxyAuthenticationRequired
QNetworkAccessCache::addEntry: overriding active cache entry 'auth:proxy-http://REMOVED@REMOVED:3130#=FDM%20TEST%20Proxy%20Server='
QNetworkAccessCache::addEntry: overriding active cache entry 'auth:proxy-http://REMOVED@REMOVED:3130'
QNetworkAccessCache::addEntry: overriding active cache entry 'auth:proxy-http://REMOVED:3130'
onProxyAuthenticationRequired
QNetworkAccessCache::addEntry: overriding active cache entry 'auth:proxy-http://REMOVED@REMOVED:3130#=FDM%20TEST%20Proxy%20Server='
QNetworkAccessCache::addEntry: overriding active cache entry 'auth:proxy-http://REMOVED@REMOVED:3130'
QNetworkAccessCache::addEntry: overriding active cache entry 'auth:proxy-http://REMOVED:3130'
onProxyAuthenticationRequired
QNetworkAccessCache::addEntry: overriding active cache entry 'auth:proxy-http://REMOVED@REMOVED:3130#=FDM%20TEST%20Proxy%20Server='
QNetworkAccessCache::addEntry: overriding active cache entry 'auth:proxy-http://REMOVED@REMOVED:3130'
QNetworkAccessCache::addEntry: overriding active cache entry 'auth:proxy-http://REMOVED:3130'

3. QNetworkRequest request(QUrl("https://ya.ru"));
request.setHeader(QNetworkRequest::UserAgentHeader, "FDM/6.0.0.0");
Problem is NOT happening. Log is showing:
onProxyAuthenticationRequired

I've attached a minimal reproducible example.