I'm running MW 1.28.
Two pages are noted in the array in localsettings:
$wgIframePageSrc = array( 'learningapps' => 'https://learningapps.org/watch?v=', 'YouTube' => 'http://www.youtube.com/embed/' );
When using the extension via special:Iframepage/ .... Both sources work fine.
If I use the piping route via : {{Special:IframePage|'YouTube'|path=XXX}} , the extension behaves as if no parameter was given, defaulting to whatever comes first (in the above example of $wgIframePageSrc learningapps, but if I switch position it defaults to Youtube.