GWR withdraw berth-inclusive fares in 2022, in favour of variable berth supplements.
These are only supposed to be sold in conjunction with a walk-up ticket for the journey, though this isn't mentioned on either the GWR website or the National Rail website. It's only when you try and buy a ticket on GWR's website that it becomes obvious, as the Cabin option is only shown against the walk-up fares.
It is, however, mentioned in Knowledgebase, so if the original ticket and/or Supplement was purchased at a Ticket Office they should have been aware of the change.