On my Bravia remote’s voice control and Plex app, I can search my various Plex libraries by voice.
Haven’t been able to get the same functionality from my Apple TV’s siri functions. Some on reddit claim to be able to do so, but when I follow their instructions, I have no luck.
It works in Plex like it does for Netflix or YouTube. The only reason I was having an issue was because I had turned off “Dictation” in the Apple TV settings by accident.
Are you just dictating into the Plex app’s search bar? That’s just dictation, which is a built-in feature. It’s not actually using Siri to search for anything.