I can't find the delete button for my comment, so I edited to this.
When you swipe down to access the quick panel / notification panel, do you see 2 tabs under the quick panel icons labelled "media" and "devices" ?
Have you double checked the notification settings are set to show all notifications ?
Settings > Notifications > Advanced Settings and make sure the option is set to show all ?
I know you said the YT app itself is set to show all notifications , but have you gone in to the phones own settings?
Otherwise, do you have a spare android device you can log into with the same Google account and install YT app to check it's behaviour on a different device ? then that way you are checking the app with the exact same details / log in , etc
Unfortunately I am all out of ideas then mate, sorry.
And I haven't had a new update for my device yet, and currently no issues as such (yet)
YT is a google app and I find they are the most buggy , so I usually now just use YT web browser version and set the browser to desktop for background play