explicitly-allowed-ports commandline argument does not work

When e.g. visiting localhost:6666 with Chromium, it shows a ERR_UNSAFE_PORT error (and QtWebEngine does too).

With Chromium, passing --explicitly-allowed-ports=6666 makes that error go away (showing ERR_CONNECTION_REFUSED instead). However, the same passed to a QtWebEngine project (like simplebrowser) has no effect.

Some context:

• https://jazzy.id.au/2012/08/23/why_does_chrome_consider_some_ports_unsafe.html
• https://superuser.com/questions/188006/how-to-fix-err-unsafe-port-error-on-chrome-when-browsing-to-unsafe-ports