They appear to have removed the second regular sleeper (the 31 car line) from the consist for the entire summer after having sold space in it. This happened before the Southwest Chief derailment and was due to a general serviceable car shortage, which the derailment only made worse. They appear to have made this decision in late May/early June. Compounding an already bad situation, they are not proactively informing people until just a couple weeks before their trip.
If you have not been notified and hold space in 731 or 831, I encourage you to check your booking on the Amtrak app. If your Empire Builder trip segment is missing the QR code, it means your sleeper has been cancelled but they haven't told you yet. In that case, call immediately. You may be able to get available space in the remaining sleeper, or otherwise adjust your plans (difference day, etc).
I find Amtrak not notifying people of a known situation until shortly before their trip unconscionable.