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: **Before Poweroff** When I then press the power button, and power it back on after < 10s, the list is the same. But if I wait longer than that: **After Poweroff** ...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...