Hey there,
Is the Optimized Battery Charging on?
you check that by going to Settings > battery > batter health > Optimized Battery Charging.
if it on This will stop charging the phone when it reaches 80% it will pause and continue before your waking time according to what the device has learned from you, for example if you connect your phone in the charger before bedtime at 9:00 pm and wake up at 6:00 the device will learn this and will organize the charging process based on your daily routine for more information see ☞ About Optimized Battery Charging on your iPhone - Apple Support
If this option is not on, you will need to answer these questions.:
Have you changed your battery before?
if so If so, have you changed it in one of Apple's stores or authorized distributors?