Cannot use accept acceptGitlabMR in a downstream job

## Issue

### Context
- **Gitlab plugin version**: 1.5.11
- **Gitlab version**: 11.8.0-ee
- **Jenkins version**: 2.150.3
- **Job type**: Pipeline

I am trying to launch an acceptGitlabMR from my pipeline job. My job is triggered from another job, and then has the Cause (GitLabWebHookCause) wrapped inside an hudson.model.Cause_-UpstreamCause, which prevents the plugin to access the Cause data. Is it supposed to work this way or is this a missing feature ?

I am not sure of this diagnostic, the step is mentionned in the jenkins logs, but there are no error logs at all.