PostgreSQL Blue/Green blocked by incompatible max_worker_processes

I'm trying to set up a blue/green upgrade of a PostgreSQL cluster from 11.21 to 15.4. During the process I've had several incompatible parameter issues, which I was able to fix by updating the parameter group of the green cluster.

However, the final issue I had is this: Creation of blue/green deployment failed due to incompatible parameter(s): max_worker_processes.

On blue instance the parameter has a value of 30, on green instance also 30.

What could be the reason for the incompatability?