`concurrency` with properties now causes "syntax error"

Problem

By making a trivial change to an existing `test_build.yaml`, GitHub Actions complains about a syntax error in the `concurrency` property. [code block] - I'm running in the github.com Actions environment, not my own. - If I re-order the properties it remains broken. - If I remove the properties from `concurrency` it works again. - I've validated that the yaml is valid yaml. So the error is confusing and unhelpful. Note: This exact yaml file has been working for months. The only change I made was to rename a job. Broken Example [code block] Working Example [code block]