Helix Job Monitor reports no Jobs submitted

I kicked off [a run of Roslyn-CI](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1577610&view=logs&j=4f7833bd-864e-5add-897e-4e99d9fca970&t=aacd478c-8183-55d2-4649-77968310b9a4&l=65) from a branch instead of through a PR. The Job monitor reported
> ##[error]No Helix jobs were submitted by this stage in any attempt. Stage: 'Windows_Debug_Tests', BuildId: 1577610.

This is despite the submit job [queuing helix work items](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1577610&view=logs&j=b9169d76-d35a-5573-bbdc-8153494e12ae&t=07c8f947-841e-574e-8ec4-1211f9d80132&l=209).

The [normal PR CI](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1577578&view=results) for this branch had no trouble monitoring the Helix work.

Start by comparing the linked Roslyn-CI branch build logs with the normal PR CI results, focusing on the Windows_Debug_Tests submit and job-monitor stages. Trace why queued Helix work items are not recognized for the branch-triggered build; done means the monitor reports the submitted jobs without affecting the existing PR flow.

Written by the indexing model from the issue text.