Background download fails with "Lost connection to background service"

For an upcoming update to one of our apps, we are looking to download fairly large directories (as tar.gz) via silent content-available push notifications.
It seems like this all works nicely:
- The notification is delivered and a background download is initialised using

backgroundSession.downloadTask(with: urlRequest)

- The app delegate method is called once the download has finished to resume the tasks
However, as soon as we create a replica download session in order to handle the tasks, the request fails with error:
"Lost connection to background transfer service".
The strange thing is, the completion handler (Internal closures, we are definitely using the delegate for download tasks) that is logging this error to our log file is on the original request controller which made the request, which IMHO, should have been removed from memory when the app entered the background?
Is the reason that we're seeing this error, because we are trying to start a second URLSession with the same configuration identifier as the one that is somehow being kept alive with our app in the background? Or is it something else funky we're doing?
The original request controller code can be seen here:
https://github.com/3sidedcube/ThunderRequest/blob/f1d5e1a3026b095d48bf0ba97bb356a283c6422e/ThunderRequest/RequestController.swift#L633
The class we use to handle the events for the background session can be found here:
https://github.com/3sidedcube/ThunderRequest/blob/master/ThunderRequest/BackgroundSessionController.swift

And the calling code of these two can be found here:
https://github.com/3sidedcube/ThunderCloud/blob/8efff2eb21b9062359824c91be39771acbf9f9c3/ThunderCloud/ContentController.swift#L802
This is the line which logs our error:
https://github.com/3sidedcube/ThunderCloud/blob/8efff2eb21b9062359824c91be39771acbf9f9c3/ThunderCloud/ContentController.swift#L968
And this is the extract from our log file:

2020-04-07T15:15:27Z [INFO] | com.threesidedcube.ThunderCloud (AppState) | applicationWillResignActive
2020-04-07T15:15:28Z [INFO] | com.threesidedcube.ThunderCloud (AppState) | applicationDidEnterBackground
2020-04-07T15:15:28Z [DEBUG] | com.threesidedcube.ThunderCloud (ContentController) | Scheduling background updates with minimum fetch interval 16238.858587878372
2020-04-07T15:16:17Z [DEBUG] | com.threesidedcube.ThunderCloud (ContentController) | Push notification received, checking if it's a `content-available` push:
{
  "payload" : {
    "filename" : "https:\/\/s3.amazonaws.com\/dev.arc.cubeapis.com\/bundles\/1\/live\/1369324617_1586180214.tar.gz",
    "timestamp" : 1685827838
  },
  "aps" : {
    "content-available" : 1
  }
}
2020-04-07T15:16:17Z [DEBUG] | com.threesidedcube.ThunderCloud (ContentController) | content-available == 1 sending notification off to `ContentController`
2020-04-07T15:16:17Z [DEBUG] | com.threesidedcube.ThunderCloud (ContentController) | `appLaunched` called
2020-04-07T15:16:17Z [DEBUG] | com.threesidedcube.ThunderCloud (ContentController) | Checking for app upgrade
2020-04-07T15:16:17Z [DEBUG] | com.threesidedcube.ThunderCloud (ContentController) | Handling content-available notification
2020-04-07T15:16:17Z [DEBUG] | com.threesidedcube.ThunderCloud (ContentController) | Making sure notification bundle isn't after a landmark publish this app shouldn't receive
2020-04-07T15:16:17Z [DEBUG] | com.threesidedcube.ThunderCloud (ContentController) | No landmark timestamp provided in notification
2020-04-07T15:16:17Z [DEBUG] | com.threesidedcube.ThunderCloud (ContentController) | Checking notification timestamp against latest on-disk bundle version
2020-04-07T15:16:17Z [DEBUG] | com.threesidedcube.ThunderCloud (ContentController) | Downloading content-available bundle with timestamp: 1685827838.0
2020-04-07T15:16:17Z [DEBUG] | com.threesidedcube.ThunderCloud (ContentController) | Downloading bundle: https://s3.amazonaws.com/dev.arc.cubeapis.com/bundles/1/live/1369324617_1586180214.tar.gz
Destination: file:///var/mobile/Containers/Data/Application/8030474F-6348-4956-984D-41098087C1FC/Library/Application%20Support/StormDeltaBundle/
2020-04-07T15:16:56Z [DEBUG] | com.threesidedcube.ThunderCloud (ContentController) | Handling events for background url session: 3FF7CD8C-C83D-4285-912B-EAA538F05458-377-00000002B2877B71
2020-04-07T15:16:56Z [DEBUG] | com.threesidedcube.ThunderCloud (ContentController) | Starting background request controller
2020-04-07T15:16:57Z [WARN] | com.threesidedcube.ThunderCloud (ContentController) | Downloading bundle failed: Lost connection to background transfer service