Werthead

Confirmed solution: you should have two autosaves called "Point of No Return." The earlier one is before you jump in the pool of water. From there you should be able to run back to Solace Keep, fast-travel to a camp and swap your party members. I switched to Marius and Kai, then went back into the Keep. The cut scene then played correctly. I've now seen five or six people say this all worked for them.