I've been trying to troubleshoot this issue since I got my phone 7 or 8ish months ago now, which is also a Pixel 8. I happened to mention it recently in the discord chat and was pointed to this thread. I think this could be related to Battery Saver, which I just have enabled 24/7, which is how I ran my older non-GOS phones. I would still get critical battery warnings at 10, 5, 3 percent on those other phones. Several times now my phone has died mid-conversation because it doesn't give me any warning before-hand that my battery is about to go.
I'm not sure if this would be a bug that needs to be fixed or a feature request, but I would love to somehow enable low battery warnings on my Pixel 8 even with Battery Saver enabled.
Does anyone know what the correct behavior should be with low battery warnings? Currently trying to play with Battery Saver more to see if I can figure anything else out.