OK the plot thickens somewhat. With a HTTP proxy enabled (I used Charles on the Mac when simulator debugging, and tried both Charles and Proxyman on an iOS device when doing a tethered debug session), then I experience no errors with the background uploads. As soon as I turn off the proxy I start getting the intermittent "400 - Invalid Proxy" errors. I don't understand how the man-in-the-middle proxy would be doing to help it work...maybe timing or something? Here is the Charles log of the successful uploads. The "Errors" here are just the 401 - Unauthorised challenges, otherwise after auth the upload works: Details of a successful image "PUT":

According to recent posts in https://forums.swift.org/t/swift-concurrency-back-deploy-issue/53917/27 seems like a fix is in Xcode 13.3 (currently beta)?

Out of interest did you ever raise an enhancement request? Our app too would benefit from being able to retrieve it's log entries from previous executions...