Yes, I have also been facing the same issue and inconsistency, sometimes ‘Siri’ works and sometimes doesn’t and the same with ‘Hey Siri’, doesn’t make a different if saying it to the phone my watch or when wearing my AP pros.
Tried resetting the Siri configuration by going through the Hey Siri or Siri (As if you’re setting a new phone) but that didn’t do much, I assumed this was an issue from my end only, glad I wasn’t alone.
Me and my Wife gave up on using Siri all together in both 14pm & 15pm.