Mail categories aren’t on iPadOS beta 1. They’ll be there later.
They changed how Siri responses work in an earlier 18.1 beta where now verbal responses are just rare in general; with the default option I pretty much only get verbal responses if I use “Hey Siri” and I’m not looking at the screen.
That being said, Siri doesn’t read chatGPT results. It will just say “I sent results from ChatGPT to your phone”.
I’m not on an iPad, but I haven’t had an issue of results going away after a few seconds.