Battery Notification Doesn't Seem To Work

I set up a Notify rule for low battery, and checked "All". I just noticed a sensor that was 8%, while the notify trigger was 20%.

Was there an actual event from the device? You need an event for the Notifications app to "trigger." Nothing will happen with the device having already been in that state at the time the rule was created (unless/until a future event comes that again matches your criteria).

It may be good to share more information about your particular app configuration if you need more help.

Yes. The rule was in place for quite a while.

I deleted the rule already, lol. It was the only rule I had in the HE Notify app.

Being in place for a while doesn't mean this particular device sent a battery event; you would need to check Events for the device to be sure. If that is what you mean, I'd suggest re-creating the rule, enabling all logging, and providing Logs filtered to this rule and possibly the device to demonstrate the problem.

Closing since app was deleted and there is no longer anything to get help with.