For posterity: the solution for OnePlus+ Watch 2 to get consistent sleep graphs with no gaps is to join the beta channel (https://docs.sleep.urbandroid.org/faqs/get_beta.html).
After doing so (the phone app version now is 20250218, watch – 5.27; thank you @lenka-urbandroid for guiding me through it providing general support) I get no more red bars of sleep graphs. And I didn’t notice a battery usage increase for the watch as well: it’s now around 1.5-2.0% of the battery per hour of sleep. With this my OnePlus+ Watch 2 lasts 2 days with the Always-On Display feature.
Overall the setup is:
- Beta channel SAA apps both on the phone and watch.
- On the watch:
- Auto-hibernation disabled.
- “Pause app activity if unused” is disabled for SAA.
- Sensors access at all times granted.
- Auto Sleep mode enabled.