Hi all. A search for "mobile network standby battery" sent me to this thread.
My device is a Samsung Galaxy A3 dual sim, and I'm having exactly the same issues I see posted in here. For me -as for some of you- the puzzling point is: why only some owners experience this bug (on a given model)?? That really baffles me. In Galaxy A3 Oreo threads, like here, only some users report this problem.
Might it be related with double-sim submodels? In several forums out there addressing this issue, people mention their devices are dual sim.
I did one experiment: flashed RR, Gapps pico, Supersu, then booted, and didn't use the phone nor change any default setting. Guess what? Mobile network standby was absolutely normal for a few hours. But then, when I started using the device and personalizing it, the bug appeared and ditn't go away. Weird, huh? Can somebody else confirm this?
[EDIT] Bad news for all of us, if someone's following or reading this thread: Battery drain hasn't improved in LOS Pie: it's still there.