Transport Scotland say this:
Scottish Ministers have published a Prior Information Notice (PIN), on 24 June 2022, in respect of the Caledonian Sleeper Franchise. This PIN has been published due to existing provisions in the Franchise Agreement whereby the franchisee may exercise a rebasing option on or after 1 April 2022 with a number of possible resultant consequences.
Presumably one of those "resultant consequences" was to trigger a break clause whereby TS could terminate the contract. In any case I suspect Serco, in requesting this "rebasing", had said they could not continue under the old terms. If those were even on offer after their EMA -that might have reverted to the old contract, or might not.