Apple developer Joe Kun took the Apple Developer Forums and announced that Apple is re-evaluating their ‘previous decision to disable changing the playback rate for subscription content from Apple Music in third-party applications. He further concluded by stating that - "could safely enable that functionality again, just like before the release of iOS 15.4", as per the report filed by GSMArena.
Joe further said, "As such, this issue is fixed in iOS 15.5 beta 4."
Although the tech giant did not reveal the real reason for disabling the playback speed adjustment feature for the third-party apps. But as Kun posted we can further be assumed that the feature was disabled because of the bug, which has been fixed by the time of writing.
Though iOS 15.5 is still in beta version and is being developed, so those who are on the stable channel will have to wait in order to be able to start adjusting the playback speed in the third-party apps, via so using the Apple Music API.