Gitlab notification working with script from CI, but not custom script in Jenins - How to debug

Hi,
related to #691, I have the situation that using Cloudbees jenkins, notifications from jenkins to gitlab for multibranch pipelines work (roughly) like with a file in the SCM like this:

```
pipeline {
// ...
properties([gitLabConnection('Gitlab connection')])
updateGitlabCommitStatus name: 'VERIFY', state: 'pending'
gitlabCommitStatus(name: "VERIFY") {
sh("mvn --errors --batch-mode verify")
}
}
```

But when I copy&paste the changes to the Jenkins configuration of the same jenkins job without any other change, no more notifications are sent to gitlab.

So I assume the action of checking out the repo for the sake of reading the Jenkinsfile to use has an effect on later calls to `updateGitlabCommitStatus` and `gitlabCommitStatus`.

How can I further debug this, and is there something else I can do to make notifications work with a custom script?