As you apparently didn’t read the thread you posted to before reading it, I will repeat the answer I posted:
Maybe because you are using it constantly, as its new. Or it hasn’t finished its background process of indexing content after the content move from the old phone. Or you’re not following best practices for charging.
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
If you charge overnight the battery should last through the day until the next nightly charge. If it does then it is working normally.