Any s21 uses noticed overnight battery drain? At 10pm last night the battery was at 38%, and at 6.30am it was at 5%. There was no wifi or mobile Internet connection and its not been used so it is weird. This is twice I've noticed this, but if it's at 70% or higher I don't think it drains as much.
I would also recommend a full reset and start phone from scratch (no transfers from old phone) as some apps like Facebook, chrome, WhatsApp don't seem to be playing nice with the S21 series..