Updates do not reliably reach the Live environment

[pghk]

To reproduce:

1. Create a new custom Drupal (7 in my case) upstream in Pantheon
2. Create a new site mysite in Pantheon using the custom upstream
3. Initialize mysite’s Test and Live environments in Pantheon
4. Commit and push a trivial change to the custom upstream repo
5. Wait for the update to become available in the mysite’s Pantheon dashboard for the dev environment
6. Run the plugin: terminus site:update mysite --message="Upstream Deployment Test" --skip_backups

Expected result

The code change would be merged into mysite’s Dev environment, then deployed to the Test and Live environments

Actual result

The change only makes it up to Test:

 [notice] mysite Starting
 [notice] Applying 1 upstream update(s) to the dev environment of mysite...
[============================] 18 secs 8.0 MiB [notice] Applied upstream updates to "dev"
 [notice] mysite: dev drush updatedb
 [notice] mysite: dev drush clear cache
 [notice] mysite: test deploying updates
 [notice] mysite: test drush updatedb
 [notice] mysite: test drush clear cache
 [notice] mysite: live has no deployable code
 [notice] mysite: live drush updatedb
 [notice] mysite: live drush clear cache
 [notice] Completed Upstream Update for mysite
 [notice] Started: 2018-05-09 18:15:16
Ended: 2018-05-09 18:15:53
Total Time: 0 hours 0 minutes 37 seconds