Battery drain on galaxy s21 plus

Give the phone a chance to adapt to the new update.  Battery optimizations take time to settle after an update.  It should go back to close to normal for the device after a week or so of moderate usage.  

Software updates can result in different device behaviours for the end user. I've experience battery drain after an update on different phones and I've found factory resetting (after backing up) and restore solves the battery issue every time.