I have investigated a report of Airplay not working in Stage Traxx. When you tap on the Airplay selector in Settings, you can select an Airplay target, but it only keeps spinning and never activates.
This is an issue with iOS that affects many apps (some still work though, i.e. Spotify). See here for more information: https://discussions.apple.com/thread/256029988?sortBy=rank
If you depend on Airplay, don't update to iOS18.4!