I got the following response from Apple for my feedback report. Following these steps did work for me: Engineering has provided the following information regarding this issue: This error occurred because a different Apple ID accepted the tester invitation for ***@.com, and a tester can be linked to only one Apple ID. To unlink the two Apple IDs, take the following steps in App Store Connect: Remove the impacted tester from the All Testers tab in TestFlight for all apps. Note: Testers in the Deleted state aren't fully removed from TestFlight until their last installed build expires. Re-invite the tester. Make sure that the tester is logged in to the TestFlight app with the correct Apple ID when they accept the new invitation. For details about how to manage testers in TestFlight, go to App Store Connect Help. https://help.apple.com/app-store-connect/#/devfd291ca86

I can also reproduce this issue. Reported to Apple: FB9994703

Did you find a solution @jeromedanthinne? I created a Feedback Report and a Code-Level support ticket for this issue and 1 month later I still have no feedback at all from Apple.

I have just checked the values from the mediaSubTypes property. Sadly all that I got are a couple of empty arrays. (lldb) po playerItem.asset.mediaSelectionGroup(forMediaCharacteristic: .audible)?.options.map { $0.mediaSubTypes } ▿ Optional<Array<Array<NSNumber>>>   ▿ some : 2 elements     - 0 : 0 elements     - 1 : 0 elements (lldb)