How do I configure CDK to deploy to the correct region?

I'm trying to deploy a simple stack to AWS using CDK. Unfortunately though when I'm running it through my GitHub actions workflow it seems to be deploying to the wrong region and I can't work out why...

The contents of my bin\cdk.js look like (sample account):

#!/usr/bin/env node

const cdk = require("@aws-cdk/core");
const { CdkStack } = require("../lib/cdk-stack");

const app = new cdk.App();
new CdkStack(app, "CdkStack", { account: "000000000001", region: "eu-west-1" });

This looks correct to my eyes following the docs. I haven't defined anything anywhere else as far as I'm aware. If I run a cdk bootstrap the CloudFormation is generated. The only reference to region is:

"environment": "aws://unknown-account/unknown-region",

Am I missing something totally obvious here? The way I've got it configured in my pipeline is simply using:

            - name: CDK Deploy
              env:
                  AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID}}
                  AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY}}
              run: |
                  npm install -g aws-cdk
                  cdk bootstrap
                  cdk deploy --require-approval never

You can set the default region as an environment variable in the pipeline configuration:

- name: CDK Deploy
  env:
    AWS_DEFAULT_REGION: eu-west-1
    ...
  run: |
    npm install -g aws-cdk
    cdk bootstrap
    cdk deploy --require-approval never