I just got my S21 FE back in October (purchased new from authorized vendor) and I'm not at all impressed with the battery performance. My 4-5 yr old S10 lasts longer than the S21 FE.
Surprised a new model was available the battery on a new phone takes 3 weeks to learn so the first week is worst about 12 days use you should start to see improvement
You bought a new 5 year old phone. Even in storage and not active a battery will deteriorate. The battery will improve in performance as it learns your usage patterns. Most likely you are using it more during setting it up and customizing, so that doesn't help with learning either, not to mention you probably have several catch-up updates til you get to the current OS/OneUI, and each update uses a lot of battery power.
