Airplay to 3rd Display Stopped Working with Catalina

Since moving to a brand new Macbook Pro running Catalina, I am no longer able to use a 3rd display the way I used to before the switch. I have a TV/monitor connected to an Apple TV, and I used to use Airplay to send a video source to that display while using 2 others as a primary/secondary (all are connected to a hub). I am able to get the 3rd source activated, but can only 'mirror' one of my primary displays, even when I specify "use as separate display". Is there some kind of new limitation in Catalina that could be causing this? Any advice appreciated.