Charge drop
I plugged in my iPhone 12 and left it to charge for two hours the charge reached 100% and dropped 30% without use.There were no backup apps and no refresh.
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Did you see what it showed for Settings > Battery ? That gives the last 24 hours of battery charge level, and whether or not it was connected to power. There can be gaps if the iPhone was powered off for whatever reason.
About the battery usage on your iPhone, iPad, and iPod touch - Apple Support
This is the photo from the example. There will be a bar (or bars) at the bottom of chart that correspond to the time that it was connected to a power source.
Perhaps you can have a look at this before the time period you had issues with goes away.
