It's the AW5' memory management bug. If you got more than 5 apps going in your 'dock/favorites' it gets laggy and things stop working--a lot like my Galaxy S5 does.
Currently I got 6 apps going in the dock, and my watch struggles to send a SMS via iCloud. Just sits there with the green line up top, fails twice, third time works.
Other times Siri just quits trying anymore, she won't do anything but error out "I can't take requests now" (workaround I use Siri via my AirPods)
A reboot is the onlty real fix--until it runs outta RAM again. Process repeat. I said this before in the forums only to get shot down, the AW5 sucks. It looks nice, and I haven't had the battery issues that everyone complains about, but functionally it is far worse than my AW3. My 3 was faster, zippy, and nothing crashed or stopped working--even with 9 apps in the dock going. The ECG never has worked on my 5. It always says 'inconclusive' I think because my normal heart rate is over 100 BPM (has been since I was 9.)
Mine never got updated (I refuse updates since they always change something that I hate adjusting to or break stuff) so it's not really software updates causing it. My AW3 however I did update until I figured out how to stop them, but it never has these issues. there's a serious QC control issue with the AW5. I got mine in Nov 2019.