I've noticed many times (I think it happens every, or at least almost every, time an intercity line finishes a route) that passengers just disappear during the journey. For example, a train with 34/34 passengers arrives at the destination with 31/34 people, or a L'Obeissante arriving with 5/6 passengers when it started with 6/6.
What happens here? Are passengers dying during the route (seeing how they're all individually simulated, I don't think it impossible - but a few every journey?)? Or are they falling out at steep curves? :D

That is happening to everyone :)

Even in TransportFever the feature is confirmed (asked in the Twitch chat when they streamed).

Well, at least I'm not the only one.
Maybe they're jumping out of the vehicle because they can get to their destination faster this way :). It's just a pity they only pay when they get off, not when they board.