custom notification sound not playing when in background mode

Our app uses custom notifications sounds which have been working fine when the app is open and when closed or in background mode. But somehow on a recent update they have stopped working when in background mode. The notification is received but the default sound is played instead of the custom sound.

xCode 11.6
iOS target 11.0

Testing on iPhone 7 and iPhone X (both 13.6.1)

The sound files are all located at {Project}/Resources/ and were added in xCode with "Copy items if needed" checked and "Create groups" checked.

Really need help with this as our app functionality highly depends on the custom sounds!
Through some additional research and checking with other developers this seems to maybe be something caused by new or different APN requirements that Firebase/FCM has not updated on yet? Please see this post https://stackoverflow.com/questions/63420587/fcm-custom-sound-not-working-on-ios-when-in-background-mode

Is there additional detail we could find out about?
After communicating directly with Firebase/FCM they maintain that there is no issue on their end causing this and are suggesting Apple may have made a change on your end that they are unaware of? Are there new APN payload requirements that would cause custom sounds to stop working when the app is closed or in background mode?
custom notification sound not playing when in background mode
 
 
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