Same issue. My recent testing confirmed your issue and found a work around as below.
I also wonder if Apple has recently dropped the default "Artist" display from the Lock Screen when using "AVPlayer.currentItem?.externalMetadata" method or it's an outstanding bug fix at Apple ? Anyone know ?
using the traditional "MPNowPlayingInfoCenter.default().nowPlayingInfo" + "MPRemoteCommandCenter" + "AVPlayerViewController .updatesNowPlayingInfoCenter = false".
Result "Artist" is displayed correctly in the iOS Lock Screen. Agreed - more code required.
using "AVPlayer.currentItem?.externalMetadata" + "AVPlayerViewController .updatesNowPlayingInfoCenter = true (ie: default)".
Result: "Artist" is not displayed in the iOS Lock Screen.
Thanks,
Jeff