Battery
My iphone is shut down while battery has 20 present and when the battery is completely dead it shows 20 present when connect to the charger( starting to chat from 20 present not 0)
iPhone XS Max, iOS 15
Perhaps your battery is reaching (or has reached) the critical point of its life, and can have unpredictable behaviors.
Contact Apple Support directly (they have tools to run remote diagnostics, battery included) using this link:--> Get Support.
Regards
Giulio