Before going any further please update your apps! Both YouTube app and YouTube Studio apps.
TROUBLESHOOTING STEPS NOT COVERED IN THE VIDEO:
Youtube added "Scheduled Digest" in the notifications section. Turn that off, because that may be the reason why you're getting 'occasional' notifications.
If that is already turned off, then the problem may be with the bell settings of that particular Youtuber.
You can check to see if his is the problem by going to:
From there, find your favorite content creator and make sure that the bell is filled in the with the grey color as well as having the vibration icon around the bell. (This means that you should get a notification for every single thing that they do).
Never miss a notification again. For now. Thanks for watching!