I am testing integrated composer for a D7->D9 migration. I can change either composer.json in the upstream-configuration or the project root to add drupal modules and this builds fine locally. When I commit these changes it appears that it builds successfully on Pantheon and I can access the site. However, whenever I click on ‘check now’ to find upstream updates I always get the message
Something went wrong when applying updates.but with no ‘View Log’ link as described in the documentation. There appears to be no way of seeing what went wrong with the upstream updates. I tried the the suggestion of merging the upstream changes using
git pull https://github.com/pantheon-upstreams/drupal-project masterbut this merged without any conflicts.
How can I find out what is going wrong with the upstream updates?