At least on my US model (TA-1049), Nokia included "DuraSpeed," a MediaTek service that kills apps that run in the background, in the Android P update (build 00WW_3_180). It can be disabled through adb.
adb shell settings put global setting.duraspeed.enabled 0
Hoozah! My 3.1 is usable again.
HMD, if you're listening, you really need to stop packaging these task killers. They fundamentally break the behavior of Android, causing missed notifications and nonfunctional alarm clocks, sports trackers, automation apps, etc. You will lose the Western market if your phones keep doing this by default. At least include a DuraSpeed control in the Settings app so we don't have to use adb to turn this crap off. The 3.1's 2GB of RAM is more than sufficient to run things in the background.