For autosync (including instant upload) to work apps must run in the background. If manual sync works but autosync does not, your device probably is kiling background apps. On devices from OnePlus you need to modify some system settings.
“Battery optimization”: this is the so called “doze mode” which prevents certain tasks from running if your phone has been sitting idle for a while. Furthermore apps which users do not open for some days are considered unused and are suspended. This is certainly not good for background sync apps. You need to turn off doze mode. There is a section in the system settings Battery > Battery optimization. Find our apps there and change their battery optimization setting to “Don’t optimize”.
OnePlus ignores “autostart” app flag. Apps are not automatically launched in the background when you turn on your phone. You need to manually launch them at least once. In older OxygenOS versions (OnePlus flavor or Android) there is an option in the system settings to allow specific apps to autostart, but it’s gone in their newest version.
OnePlus kills background services if they are actively running for longer than a few seconds. That means our sync apps are killed in the middle of autosync operation if it takes longer than 5 seconds or so. Of course once killed, they cannot schedule any next autosync. You need to lock down apps to prevent them from being killed by the system. Open our apps. Switch to somewhere else. Tap on the device “Recents” buttons to display the recent app list. There is a lock icon next to each app in the list. Tap on that lock to lock down the app.