NOTE: This is an anecdotal workaround (or solution - whatever you wanna call it). I do not know the actual issue. A few weeks ago I was reminded of something and I tried it out, and it worked! This may or may not work for you. No guarantees.
Also, there is no risk involved actually but, if for some reason things go wrong, IT IS ON YOU. So proceed with caution.
Okay, all you need to do is to go to the recovery mode and, wipe the cache partition.
Double...triple check that you have picked the correct option (Wipe Cache Partition) and nothing else. Do not be over-confident. Accidents happen like this. If you accidentally wipe data, it is going to be a sad day (for most). Trust me, it happens more than you imagine, even when you have to enter commands manually.
Technically, you do not need to do this (OTAs clear cache after successful installation - I reckon what and why of this can be found on AOSP documentation). Doing this manually does not have any benefit. However, there are times when things get mixed up and, something like this becomes helpful. So, you don't need to do this every single time. I have done this on almost every phone after an update that introduced some weird bugs or glitches. I used to perform this task since the earlier days of Android. It solved a few things, while it did not fix a few other things.
I faced the accelerated battery drain issue after the M307FXXU1ASJ1 update on 17th October. After using it for a few days I was reminded to wipe the cache. I did, and my issue was fixed.
The most recent update (...SJ2) on 30th October was totally fine.
I saw a lot of posts about the battery and I wanna say that...this phone has 99 problems but the battery ain't one. So a few points:
Again, this phone has many issues. I'm not 100% satisfied. But the battery is not one. I hope wiping the cache will do the job for you. If not then...good luck!