Same issue. I think it started for me with iOS12.1 and I still have it on 12.1.1 betas. It usually works after a reboot but then stops.
I only use Siri for a shortcut I have to translate what I say into another language. Weirdly, the setting to start translating after a pause works, but it sends nothing to be translated. In other words, it seems as if Siri is picking up the fact I was talking and had then stopped talking, but didn't actually transcribe anything I said. I've assumed it's a bug. it's been mentioned on here before I believe in one of the iOS12 beta threads.