Earlier quoted context omitted.
Youtube used to do that, until at some point they added the “bell” feature and converted all subscriptions to subscriptions-without-a-bell which means only some videos get put in your feed. Of course, if you go the extra step to click the bell icon you get the notifications like usual.
Even the bell is no longer reliable.
A quick check shows that every video posted by one of my subscribed channels (in the last week) appears in that feed. If that's not happening it sounds like some kind of bug.