This is related to SERVER-101443. When a PR message validity check fails, it provides the following diagnostic message in the log: 

If you are seeing this on a PR, after changing the title/description you will need to rerun this check before being able to submit your PR

However, this message doesn't explain how to rerun the failed check. If we can clarify it in the log message by suggesting to rerun the failed validate_commit_message task in Evergreen, it will help to avoid a lot of confusion (like I experienced myself).