This is obnoxious since I tend to watch series of documentaries, and I prefer to watch many episodes in a row. Stuff just poofs out of existence.

The feed on the home page is recommendations. That may, by chance, include things you're subscribed to but it will also include other recommended content and possibly not recommend things you are subscribed to as it's not meant to be a second copy of the subscription feed.

If I subscribe to a youtuber with a back-catalog of several years of videos, I want recommendations of those prior videos, not only their brand new ones.

And yes, there is the subscriptions feed, but it only shows new videos from subscribed-to channels; it doesn't function as a 'recommended' list for your subscriptions, which is what I miss and want.