I'm using Falkon 3.0.1 with QtWebEngine 5.11.1 on Gentoo/Linux. Geolocation is still not working. For example, https://html5demos.com/geo/ will keep forever on "checking" (on private mode it will return "failed", even with the do not track flag disabled (since there's still no web inspector (...), can't tell anything else about this). On the same machine, geolocation works on Firefox 52. I guess this has to be something related to upstream, though, because it seems Falkon is not the only Qt browser where geolocation is not working (https://www.reddit.com/r/qutebrowser/comments/8fquv8/geolocation_doesnt_work/) despite support for it was added on qt upstream since 5.5 (https://doc.qt.io/qt-5/qtwebengine-features.html#html5-geolocation).
It works for me with QtWebEngine 5.12. It needs some external location provider, like GeoClue.
As a matter of fact, Qt 5.11.3 was added yesterday to Portage's unstable tree and with it this worked - So I'm changing this from "Resolved worksforsome" to "Resolved upstream".