I’m having a similar issue. For some sites, like Twitch, it will only cast the audio (Safari does the same, sadly). For YouTube, any video will cause the AirPlay device to load infinitely and won’t even cast audio. I waited until I got a new iPad to test with a fresh installation and more default Orion settings/no extensions but to no avail.
Testing in Safari, YouTube will cast the video and audio to the device without issue. I’ve tested with a random assortment of YouTube videos and have yet to find one that works in Orion.
Orion 1.3.8 (13) (WebKit 8618.3.11.10.5)
iPad 8th Gen, iOS 17.5
iPad Pro M4, 17.6.1
LG TV running AirPlay 2