Sometimes, when Amtrak re-issued the ticket for some reason, I have had what the OP described. For example, if I noticed the fare had decreased and I called to get the cheaper fare, Amtrak would change my room but the new ticket would show only the new accommodation but not show the pricing information. Similarly, if I had booked a roomette but the bedroom price dropped, I might call to switch, and the new ticket would omit the new price.

I'm not saying this happens all the time or even usually, but I have received a new ticket without the fare information.