Well you said it worked before and even with 8.9 it does not this is strange, and maybe confirm there’s something else changed somewhere.
Anyway your logs:
10-13 16:36:54.374 1350 21970 D MediaSessionService: Sending KeyEvent { action=ACTION_DOWN, keyCode=KEYCODE_MEDIA_PREVIOUS, scanCode=165, metaState=0, flags=0x8, repeatCount=0, eventTime=148946525, downTime=148946525, deviceId=22, source=0x101 } to the last known PendingIntent PendingIntent{1f2f0e5: PendingIntentRecord{ff487ba com.google.android.youtube broadcastIntent}}
10-13 16:36:54.378 1350 21970 D MediaSessionService: Sending KeyEvent { action=ACTION_UP, keyCode=KEYCODE_MEDIA_PREVIOUS, scanCode=165, metaState=0, flags=0x8, repeatCount=0, eventTime=148946525, downTime=148946525, deviceId=22, source=0x101 } to the last known PendingIntent PendingIntent{1f2f0e5: PendingIntentRecord{ff487ba com.google.android.youtube broadcastIntent}}
10-13 16:36:54.398 1350 1482 I ActivityManager: Start proc 30828:com.google.android.youtube/u0a186 for broadcast com.google.android.youtube/com.google.android.libraries.youtube.player.ui.mediasession.MediaButtonIntentReceiverProvider$DefaultMediaButtonIntentReceiver
Shows that now the events are sent to Youtube …
The only suspicious thing from your logs is:
10-13 16:36:54.269 3384 3384 D SAMediaEventCommander: onPlaybackStateChanged(1)
10-13 16:36:54.269 3384 3384 D SAMediaAppResponseHandler: verifyWaitResponse(org.leetzone.android.yatsewidgetfree, 3): '' ---> false
But I have no idea what SAMediaAppResponseHandler is and can’t find any single link when Googling that, so it’s definitively not something standard.
Normal logs on any of my devices, event are send to Yatse.
2019-10-14 09:10:03.237 2051-3194/? D/MediaSessionService: Sending KeyEvent { action=ACTION_DOWN, keyCode=KEYCODE_MEDIA_PLAY_PAUSE, scanCode=0, metaState=0, flags=0x0, repeatCount=0, eventTime=162750620, downTime=162750620, deviceId=-1, source=0x0, displayId=-1 } to org.leetzone.android.yatsewidgetfree.debug/PlayerService (userId=0)
2019-10-14 09:10:03.237 2051-3194/? D/MediaSessionService: Sending KeyEvent { action=ACTION_UP, keyCode=KEYCODE_MEDIA_PLAY_PAUSE, scanCode=0, metaState=0, flags=0x0, repeatCount=0, eventTime=162750620, downTime=162750620, deviceId=-1, source=0x0, displayId=-1 } to org.leetzone.android.yatsewidgetfree.debug/PlayerService (userId=0)
2019-10-14 09:10:03.238 28082-28082/org.leetzone.android.yatsewidgetfree.debug V/PlayerService: PlayerService$onCreate$$inlined$also$lambda$1.onMediaButtonEvent@134: onMediaButton: Intent { act=android.intent.action.MEDIA_BUTTON (has extras) } - KeyEvent { action=ACTION_DOWN, keyCode=KEYCODE_MEDIA_PLAY_PAUSE, scanCode=0, metaState=0, flags=0x0, repeatCount=0, eventTime=162750620, downTime=162750620, deviceId=-1, source=0x0, displayId=-1 }
2019-10-14 09:10:03.239 28082-28082/org.leetzone.android.yatsewidgetfree.debug V/PlayerService: PlayerService$onCreate$$inlined$also$lambda$1.onMediaButtonEvent@134: onMediaButton: Intent { act=android.intent.action.MEDIA_BUTTON (has extras) } - KeyEvent { action=ACTION_UP, keyCode=KEYCODE_MEDIA_PLAY_PAUSE, scanCode=0, metaState=0, flags=0x0, repeatCount=0, eventTime=162750620, downTime=162750620, deviceId=-1, source=0x0, displayId=-1 }
2019-10-14 09:10:03.540 28082-28082/org.leetzone.android.yatsewidgetfree.debug V/PlayerService: PlayerService$onCreate$$inlined$also$lambda$1.onPlay@144: onPlay
Now the game would be to try to find the last version that worked but 8.9 is 8 months old already, or what else you change or use on all your devices that could prevent normal behavior.