The firmware always resets the watering level to 100% before sending out a weather query so it shouldn’t get stuck in 0% — usually if it’s stuck at 0%, it’s because the weather algorithm returned 0% to the firmware.
One example of how ETo will return 0% is if there is significant precipitation. The formula of ETo is explained in our support article:
https://openthings.freshdesk.com/support/solutions/articles/5000823370-use-weather-adjustments
as you can see if there is significant precipitation it can cause ETo to become 0%.