DemoBrowser ignores system proxy

Qt 5.8.0 seems to ignore KDE systemsettings proxy settings.

When invoked in a proxied environment, where DNS does only resolve internal addresses, WebEngine reports the error ERR_NAME_NOT_RESOLVED starting with Qt 5.8 (Linux).
Proxy configuration had been done via KDE (systemsettings > network > proxy > use proxy auto configuration url)

This may be reproduced with the "demo browser" example.

Qt 5.7 version asks for proxy password and after that displays the page of http://qt.io/ .
Qt 5.8 version just shows error page.

(After entering a single working proxy in the local settings of demobrowser, it works, but this is not the intended way when needing to use a complex proxy script with different proxies)