I don’t have any problems sending audio or audio/video (eg., Youtube) to any of my Apple TV’s (and I have several - 2nd to 4th generation).
My problem is sending audio to my Marantz M-CR510 Airplay-enabled receiver. It stopped working when I upgraded to iOS 11.0. Thereafter, neither my iPhone 7+ nor my first-generation iPad Pro could send audio to the receiver via Airplay using the music app. Once in a while I would get an error message saying that Airplay could not connect to the Marantz, but usually there was no error message, just no music.
Finally, resetting the iPhone to factory settings via iTunes fixed the problem on the iPhone, but not on the iPad. However, once I did a synch of the iPhone with iTunes to make sure my music libraries on the iPhone and my Mac were identical, the problem came back. Upgrading to 11.0.1,11.0.2,11.0.3, and finally to 11.1 all failed to fix the problem.
Your problem is a bit different, as you are experiencing audio and video (I surmise video too, since you mentioned Youtube) Airplay connectivity problems with Apple TV.
Different problems, but my suspicion is that all of the Airplay issues being reported in this thread are related to a rewrite of Airplay in order to implement the features of Airplay 2 (none of which I have tried to use or are even interested in using).
Apple is well known for inadequate regression testing that breaks previously working functionality, and I think these Airplay issues are just the latest case in point. I think they will eventually fix the problem(s), but who knows when?
One interesting fact, and one which I believe someone else has mentioned, is that not all iOS devices appear to be affected. For example, my partner also has an iPhone 7+ running the same releases of iOS as I have run, and he has had no problems connecting to the Marantz.