CI/CD Issue for Node 18.x Runtime

I am having an issue with the nodejs 18.x runtime in CI/CD.

Despite specifying the Node 18.x in my serverless.yml:

provider:
  name: aws
  runtime: nodejs18.x

I am seeing Node 12.x selected in the build:

Build started at Thu Jan 19 2023 17:29:28 GMT+0000 (GMT)

> node --version

v12.22.11

Specifying Node 16 in serverless.yml yields the correct build environment, so I am a bit confused.

Any ideas what might be happening?