Can't embark on merge train: `Unable to update: user X is unknown.`

Expected Behavior

Github users can use a Github label to get their PR embarked on a merge train.

Actual Behavior

Here's the error message we currently get:

This pull request cannot be embarked for merge

The merge queue pull request can't be updated
Details:

Unable to update: user MozillaNoah is unknown.

Please make sure MozillaNoah has logged in Mergify dashboard.

Source: https://github.com/mozilla-mobile/firefox-android/pull/1217/checks?check_run_id=11997595835

Steps to Reproduce the Problem

1. Have a user part of our Github organization mark a PR ready to embark on a merge train
2. Wait for CI to complete
3. When CI is ready, PR gets the error message above ⬆️

I'm not 100% sure but this looks like a recent regression. No one on the team has ever got this problem.

cc @rvandermeulen

Specifications

• Pull Request URLs:
  • https://github.com/mozilla-mobile/firefox-android/pull/1217
  • https://github.com/mozilla-mobile/firefox-android/pull/1234
  • we had to manually rebase the following:
    • https://github.com/mozilla-mobile/firefox-android/pull/1204
    • https://github.com/mozilla-mobile/firefox-android/pull/1232
  • this one did work though https://github.com/mozilla-mobile/firefox-android/pull/1228
• Mergify Config URL: https://github.com/mozilla-mobile/firefox-android/blob/2d0ce92593ecb04e24397c3947d1c8862c76041f/.mergify.yml

Start with the Mergify configuration in .mergify.yml and the linked GitHub check runs and pull requests, comparing the cases that failed with the one that worked. Trace the merge-train update path that produces the unknown-user error; done means organization users can embark their pull requests without this failure and the listed scenarios behave consistently.