Battery health heavy drain

The day to day doesn’t mean much, as it isn’t always linear. I already told you what matters: HOW LONG HAS IT BEEN SINCE THE PHONE WAS NEW? If the phone was new 15 months or more ago then the battery is fine.

There are things you can do that will cause the battery to lose capacity faster, such as routinely draining it below 10%, or worse, until it shuts off. Or never letting it reach full charge. Or following some of the fake advice in Youtube videos. But there are things you can do that will minimize capacity loss.

The absolute best way to get maximum use on a charge, as well as slow the decline of battery capacity long term is to enable Optimized Battery Charging (Settings/Battery/Battery Health) and charge the device overnight, every night. The battery will fast charge to 80%, then pause. During the nighttime pause the phone will use mains power instead of battery power, allowing the battery to “rest”, and thus reducing the need to charge the battery quite as often. The phone will resume charging to reach 100% when you are ready to use your phone; it will “learn” your usage pattern. If you enable iCloud Backup (Settings/[your name]/iCloud - iCloud Backup) the phone will back up overnight also, assuring that you can never lose more than the current day’s updates. Here's more information→About Optimized Battery Charging on your iPhone - Apple Support

There is no bug, but there may be a problem with your phone. How long has it been since the phone was new? Battery health on average will drop about 1% a month from the time the phone was first activated.