I consistently hear people on YouTube complain that the subscribe button doesn’t do anything for viewers, now that channel notifications are controlled by the bell. But it does do something: it puts the videos from that channel in your subscription feed, which is readily accessible on all versions of YouTube. So why do people act like it doesn’t exist? I think it’s super convenient, especially if you’re subscribed to a ton of channels and don’t want your notifications feed flooded with new videos.
Items temporarily disappear from sub page due to distributed nature of YouTube. It has too much information and too many users to work in a traditional way. This is not an “algorithm”, it’s just a synchronisation issue. It happens on all huge platforms. People frequently “disappear” from your friend list on Facebook and then reappear a couple of days later.
YouTube added a bell to reduce synchronization issues. Bells are temporary notifications, not persistent data, so it is handled completely differently.