So, I had an interesting conversation with YouTube about this recently. I was told that the Subscriber system was never designed so that subscribers will be notified of every new video that every channel that subscribe to posts. There are "thresholds" in place so that YT does not "spam" subscribers with too many videos.
I was just as surprised as you are right now. I ALWAYS thought that subscribing would mean that I would see all of my subbed channels new videos in my subscribed feed. I kind of thought that was what it meant when I subscribed! It seems that because people subscribe to large volumes of channels, YT not only decides what to show you on your homepage, but on your subscribed page!
That leads us to the notification "bell." As far as my YT rep knows there is no "threshold" on this system. So if a person has the bell checked for 1,000 channels they will get notifications for all 1000 of those channels whenever they post a video.
I feel like Jerry Seinfeld at the rental car booth when he's confused about the reservation process. "If I reserved a car, how do you not have a car for me?" If I subscribed to a channel, why are you not showing me all of their videos in my subscribed feed?