Hi, I have the exact same problem (latest iOS 14.5.1, all apps are up-to-date, device has been restarted, tried it both on wi-fi and cellural etc.). Although Siri still executes the task prompted, (s)he stops in the middle of the sentence when giving a verbal response.
Extra info: it works fine on my Watch (Watch Series 6, latest watchOS version), the problem only occurs on my phone (iPhone 11). Interesting enough, Siri works totally fine on my work phone (also iOS 14.5.1, but an iPhone 8), so it may be model-related.
Can you please fix this? It is really annoying.