Just had the Plex (2025.15.0 (1053) iOS update roll out to my iPhone 16 Pro with 18.4.1 iOS. Got forced to do a manual update of my Plex server on QNAP. Now I can seemingly create a cast connection from my iOS Plex app:
but when I go to play any content, nothing happens:
It looks like it is playing on my phone, but the progrma never appears on the TV.
If I manually navigate via my computer using Chrome, I can trigger casting from inside the Plex app (actual casting, not mirroring) and play content as normal.
On an Android tablet using the Plex app v10.26.0.2578 it casts as normal.
iOS app broken for actually playing content to a Google Chromecast 
Chromecast is quite old:
System firmware version: 281627
Cast firmware: 1.56.281627
