This is a very bad news.
I have no task killers,
only 'Disable Service' and 'App Quarantine' that aren't getting in touch with Xposed Additions.
For this reason I'm wondering if any other people have experienced this issue,
it's really strange that in my case Xposed Additions is being killed by Android and no one else is experiencing that..
Any ideas of how prevent Android to kill Xposed Additions?
It is likely 'Disable Service' that causes your problem. If the program has a log, it should explain what services that are killed, maybe the service in Xposed Additions has a different name than you expect or that 'Disable Service' kills more than it should?
If no log in that app, use adb logcat.