I’m not entirely sure where to raise this, the bug tracker didn’t really seem to have a relevant place either, but http://networkcheck.kde.org seems to just return the text “OK” which appears to be throwing off NetworkManager’s captive portal detection, making it appear as if all networks are behind a captive portal.
testing a few of the other NM portal detection endpoints (i.e gnome’s and the Arch linux one) they return “NetworkManger is online”.
Poking around NetworkManager docs it appears that this string is significant:
The URI of a web page to periodically
request when connectivity is being checked. This page
should return the header "X-NetworkManager-Status" with a
value of "online". Alternatively, its body content should
be set to "NetworkManager is online". The body content
check can be controlled by the <literal>response</literal>
option. If this option is blank or missing, connectivity
checking is disabled.