I had faced same battery drain issue in 6s plus after ios 14 upgrade onwards. Yesterday I wiped out (Erase all content and settings) and restored from latest backup which was backed up before wiped out. My battery drained issue has been resolved. Now phone is running smoothly without battery issue. Please note take latest backup before wiped out and restore same backup using iTunes(windows) or MAC battery drain issue has been resolved. Please try from ur end also to fix this issue. Step1) please take latest backup using iTunes or MAC. Step2) wipe out data ->settings->general-> reset -> erase all contents and settings. Step3) restore latest backup from iTunes or MAC. Hopefully this will work.