Red Dead Redemption 2's camp disappearances are in fact the result of a bug.
After about a week of reports of certain gang members being completely absent from the camp in Red Dead Redemption 2, Rockstar has decided to weigh in officially.
The bug, in case you missed it, prevents Sadie Adler, as well as John Marston's family (John, Abigail, Jack) from appearing in your camp after completing the Polite Society, Valentine Style mission in chapter two.
While players initially assumed this meant missing out on these characters' side missions, Rockstar has now confirmed that this only prevents players from seeing the characters interact with each other at camp. When they have missions for the player, they will appear regardless.
Players correctly guessed, however, the trigger that makes the bug occur in the first place. Restarting any checkpoint in the Polite Society, Valentine Style mission will indeed cause you to lose these gang members.
The only fix for now is to abandon the mission and restart, if you fail any of its checkpoints, or obviously load an earlier save. Rockstar revealed this in a new support article, and confirmed that it's working on a fix for the bug to be included in a future patch.
If you're way past that early chapter two mission, then you may as well wait until the start of chapter four - where the entire gang will be there.