This is a bug in the estimation of the battery life. It is pretty easy to see it.
An easy test to see is buggy and that estimation around 24h ( depending on the device ) remains somewhat static is this:
Enable dark mode, watch what is telling you, disable dark mode, it will recommend you to turn in ON and save 1h++, turn it on,
watch the estimation. Nothing changes.
Ignore that in the weekly ROM, what you have to do is to set up your phone the way you are using it, after that, enable USB debugging and run:
adb shell cmd package bg-dexopt-job
( search the forum for a howto )
Once you've done all that, plug in the charger, turn off the phone and charge to 100%.
Turn on and use the phone as always.