See here for introductory information about manufacturer specific issues...
OnePlus is very aggressive when it comes to killing background apps. There are a lot of different settings that may kill background apps. Fortunately, we have instructions on how to resolve the problem if you are using a OnePlus device. Unfortunately, some of these settings get reset with firmware updates. You may need to re-enable these settings on a regular basis.
- Battery Optimization
Settings > Apps > Gear Icon > Special Access > Battery Optimization > Disable
Warning: There are reports that some devices will revert this setting at random. You may need to double check that this setting is disabled.
- Simple In/Out Battery Optimization
Settings > Battery > Battery optimization > switch to "All apps" in the top right menu > Simple In/Out > select "Don't optimize"
- Lock Simple In/Out within Recent App List
You must lock Simple In/Out in the 'Recent App' list.
Open Simple In/Out, go to your 'Recent apps list' (the app switcher), and toggle the 'Lock' button on the upper right corner of Simple In/Out.
- App Auto-Launch
On some OnePlus phones this feature will prevent apps from working in the background. Please disable it for Simple In/Out. Screenshot seen below.
- Advanced Optimization (OnePlus 6 and up)
Settings > Battery > Battery optimization > (three dots) > Advanced optimization > Disable "Deep optimization"
Settings > Battery > Battery optimization > (three dots) > Advanced optimization > Disable "Sleep standby optimization
- Enhanced Optimization (OnePlus 5 and below)
Settings > Battery > Battery optimization > (three dots) > Enhanced optimization > Disable
- Recent Apps Clearing
Normally when you swipe an app away, it will only 'close it' but not 'kill it'. However, on OnePlus devices the app is being killed. Make sure you set 'Recent app management' to "Normal clear"