I also ran into this problem after updating to 13.3.1. The battery was dropping from 100% to 50% within a few hours. The battery health app showed various apps were draining such as Contacts or Maps. After a lengthy Apple Support conversation the conclusion was the dreaded Factory Reset.
Although I had a full B/U I opted to start fresh. I screenshot all my Apps for reference and did a fresh install.
Battery life has been Stellar. Actually better than when the phone was new.
I hated to do it it is time consuming but the outcome was well worth it.
I'm far from being a Specialist and did need help but Apple was extremely accommodating and tried a number of options via Screen Share.
Battery continues to hold a charge well into two days with minimal calls, quite a few texts, music playback.
I hope that helps you some. It really is nice to have a fresh phone.