Rockstar admits that there is a problem that makes certain characters disappear from the base.

A Q&A from Rockstar Games has allowed us to know that the studio is already working on correcting a bug in the Red Dead Redemption 2 camp by which some fans are experiencing the disappearance of certain characters from the base. Specifically, Sadie, Jack, John and Abigail after starting the second chapter of the game.
Apparently, this occurs when you complete a specific mission at the beginning of it, the first and not named to avoid spoilers, and after recharging a certain checkpoint. If the bug occurs, the four mentioned characters disappear and they will only be present if they have a mission to propose to us.
“We’re working on the problem and we’re working on a solution that will be included in the next update of the game,” said Rockstar, who, for now, recommend repeating the mission if we die halfway to avoid recharging that checkpoint that is precisely the cause of the problem.
It’s not the first problem experienced by the game, and Rockstar has already said it’s “working hard” to improve Red Dead Redemption 2. A title that, if you want to know what to expect, you can read our RDR2 review here.