Background apps get killed (by something other than battery optimization) All of my background apps get killed when I turn off the screen.
This is particularly annoying for my most-used messaging app, WhatsApp. 

I've looked at the obvious "Battery Optimization" option: 


Based on advice found elsewhere, I've checked that the background process limit in developer options is the default (I've never changed this AFAIK):


And yet, it gets killed in the same manner as battery optimization would. If I look at running background processes: 



When I then press the power button, and power it back on after &lt; 10s, the list is the same. But if I wait longer than that: 



...whatsapp has disappeared. 

This happens for all apps I install that have some sort of useful background feature - the Battery Optimization setting doesn't seem to do its job anymore. 

What else can I try to diagnose this?

NB0: Android 7.0 on DOOGEE S60

NB1: This just started happening some time ago, i.e., it did work propertly when I got the phone. 

NB2: I know how to use adb

NB3: I'm also fond of trying new apps and have ~400 apps installed. Ideally, I don't uninstall/disable all of them 1-by-1 to see which one is the culprit...
Go to &quot;settings&quot;, &quot;memory&quot; and turn off &quot;background auto clean memory&quot;.