if you still facing this issue follow below steps it might fix this issue
- sign out from your account
- complete erase your mobile.
- install the 16.2 using laptop.
- Now login but do not restore the data.
- switch off your mobile over night.
- check how much % was drained it will be 1-3 % only.
- if above statement is correct then use ur mobile for couple of hours after logging in the it but without restoring the data.
- now observe the battery. If this looks ok then restore the data
- now same night check how much battery was used after restoring the data.
- if battery drained too much after ur data then you know where is the problem.
- 99% of the time this will solve the issue.
one you follow this please update so that others will get confident.
I have found this process all by myself without Apple support.
no one helped me from Apple.