Gitlab builds fail normally but pass in debug mode

When running iOS CI builds through Gitlab.com, (they are running locally on my machine) my builds now seem to always fail with:

ERROR: Build failed: exit status 1

However, if I run $ gitlab-ci-multi-runner --debug run (which I am assuming puts the runner in debug mode?), the builds pass everytime.

What is the debug mode doing differently when building and how can I solve this issue?