Multi-project trigger job - "Downstream project could not be found" error while using valid project names or project IDs

We using the GitLab SaaS product.

I am trying to setup a multi-project pipeline as described here:

My trigger job looks like this:

  stage: tests
    project: 'sub-group-name/project-name'
    strategy: depend
    branch: branch-name

The project name ‘sub-group-name/project-name’ is using a correct value either by subgroup-name/project-name or by subgroup-id/project-id, and the downstream-tests job fails with the “Downstream project could not be found” error using double quotes, single quotes, or no quotes.

I am blocked and have no idea why the upstream project cannot find the downstream project when they both are in the same group. What can I do to troubleshoot this issue?

The explanation for this problem is that the sub-group-name had a whitespace character in it, and the fix was to substitute a hyphen for the whitespace in the group name value.

That really should have been in this document: