Fixed with 8.1, see https://community.phones.nokia.com/support/discussions/topics/7000011753
Well, I will give it a try as soon as it will be available in Germany as my primary phone with a MultiSIM is still my Lumia 950 XL :-).
I'm using 8.1 Oreo, It feels better than 7.1.2, I haven't observed the background applications being killed yet, But I can multi-task better than 7.1.2 now !!
The applications (Depends on what you use) seems to continue from where we left off, except a few- Like browser which reloads.