Unable to configure signoff commits in config

[mysteq]

Environment details

• OS: ubuntu-latest
• Node.js version: N/A (run via release-please-action GitHub Action)
• npm version: N/A (run via release-please-action GitHub Action)
• release-please version: 16.12.0 (run via release-please-action GitHub Action)

Steps to reproduce

1. Add $.signoff to config as per documentation of release-please-action

{
  "signoff": "foo <foo@users.noreply.github.com>",
  "packages": {
    ".": {
      "release-type": "simple"
    }
  }
}

2. Run release-please

3. Example relevant output from debug-config

....
separatePullRequests: true,
fork: false,
signoffUser: undefined,
releaseLabels: [ 'autorelease: tagged' ],
labels: [ 'autorelease: pending' ],
....

This issue is related to https://github.com/google-github-actions/release-please-action/issues/904 and also seems to have been tried to be solved here #2246 .