The screenshot displays all services (I made sure before posting). Maybe the phone was rebooted or something (I don't remember), but it was charged to 100% in the morning, and was at 20% at 11pm with 1hr screen-on time. I could make another test tomorrow or Sunday.
And yes, this even happens on a fresh GOS install with ZERO additional apps installed.
Note: I restart my device when writing this, and I noticed that before the reboot, the System CPU 'service' was respnsible for 7% of battery use. But opon the device restart, this 'service' and its battery use is no longer reported in the battery stats at all. I am not too sure what is going on here, but it seems the info reported may not be too accurate.
Note 2: Same issue was evident in Calyx and Lineage, which would indicate a hardware problem, but I am PRETTY sure stock OS worked just fine for me in May after Google issued a server-side patch (battery issue was common problem on stockOS in early May). I might flash it next week and triple-check.