This is due to the Android OS behavior. According to Android, when any app runs for a long time and there is not enough RAM, Android will kill the app in the background. That is why the app sometimes restarts or closes when you turn off the screen.
To fix this, you need to whitelist the app from Battery Optimization. Then the app will not close and your download will complete in the background. The location of this setting depends on the device manufacturer, so please check it from below.
If you cannot find your specific brand or model, please search it on the web or from your manufacturer’s website.
Samsung
Depending on the phone model, go to the smartphone’s settings and follow these steps:
- Tap on “Applications” or “Apps”
- Tap on the three-dot menu (top right corner)
- Tap on “Special Access”
- Tap on “Optimize Battery usage”
OR
- Tap on “Device maintenance”
- Tap on “Battery”
- Tap on “Battery Usage”
- Tap on the three-dot menu (top right corner)
- Tap on “Optimize Battery usage”
On the “Optimize Battery” screen, if the 
Torrent Pro app is not listed as “Apps Not Optimized”, tap on “Apps not optimized” and then tap on “All apps”. Search for “ Torrent Pro” and disable that option.
Google Pixel
- Go to your smartphone’s Settings
- Tap on “Apps & notifications”
- Tap on “Advanced”
- Tap on “Special app access”
- Tap on “Battery optimization”
If the Torrent Pro app is not listed as “Not Optimized”, tap on “Not optimized” and then tap on “All Apps”. Search for “ Torrent Pro” and tap on it, and then tap on “Don’t Optimize”.
Sony
- Go to your smartphone’s Settings
- Tap on “Battery”
- Tap on the (…) menu in the top right corner and press “Battery optimization”
- Tap on “Apps”
- Tap on “ Torrent Pro“.
LG
- Go to your smartphone’s Settings
- Tap “Battery & power saving”
- Tap on “Battery usage”
- Tap “Ignore optimizations”
- Turn “on” to ignore battery optimization for the Torrent Pro app.
One Plus, Realme, Redmi
- Go to Settings.
- Tap on “Battery”
- Tap on “Battery optimization”
- Switch to the “All apps list” (top menu)
- Tap on Torrent Pro and choose “Don’t optimize”
- Go back to your smartphone Settings
- Select “Apps & notifications”
- Tap on “See all apps”
- Tap on the Torrent Pro app
- Then “Permissions”
- Toggle “on” for Location access
Motorola
- Go to your smartphone’s Settings
- Tap “Battery”
- Tap on the menu in the top right corner
- Tap “Battery optimization”
- Tap “Not optimized”
- Tap “All Apps”
- Find Torrent Pro on this list and tap on “Don’t Optimize”.
- Go back to your phone settings*
- Select “Battery”*
- Turn Battery Saver Mode off*