This is unlikely to be something caused by muOS or the way it handles sleep, it’s much more likely to be due to how unreliable the battery level indicator is.
I’ve seen some people charge to 100% and unplug the device while on the main menu and they can literally observe the battery percentage plummet to 85% while still just sitting on the menu. Others have been unable to make the battery percentage show 100% even after charging for a considerable amount of time.
As Kirky says, it would be likely that you’d be able to play for a considerable amount of time while the indicator shows 0%. It may be that the system just doesn’t allow booting if it detects (incorrectly) that the battery level is too low.
Basically, I would just take it with a pinch of salt.
To mitigate your issue with sleep, you can try the sleep shutdown settings so that it shuts the device down after a set amount of time (eg 5 minutes) in sleep and instantly resumes your game when you boot it up. Together with retroarch auto save and load state, as well as the blazing fast boot times of muOS, this is a great option.
You can see more info on just how unreliable the battery percentage indicator is, here: Why Battery Levels Are Just a Guess and everything you want to know ( and some you probably didn't ) about charging on the H700/AXP717