I have been experiencing these exact symptoms with my iPhone 11 for about a week. I first noticed the problem with Siri's voice dropping out via the CarPlay interface, and I mistakingly assumed it was a CarPlay problem. Repaired, etc. I later realized it was a problem with Siri on the phone itself, as I could reproduce the problem with only the phone (no CarPlay) and with AirPods.
The solution for me turned out to be Settings > General > Reset > Reset Network Settings. Siri is working perfectly now. Perhaps there are problems with network caches or the like as Siri phones home for data support, via network connections.
Hope it works for you all.