No "Running" status for Matrix Jobs

## Issue

### Context
- **Gitlab plugin version**: 1.5.8
- **Gitlab version**: 11.0.3
- **Jenkins version**: 2.131
- **Job type**: Matrix

### Logs & Traces

I am not sure which are the relevant logs but will provide any necessary information that might help.

### Problem description

I have configured a Matrix Job which performs cross-platform compilation and testing. Once triggered by a GitLab commit the Pipeline status is correctly set to "Pending". It remains in this state until the Jenkins job finishes. Then it immediately transitions to "Finished". The "Running" status is skipped.

I have tried to configure the "Publish build status to GitLab" post-build action with and without the Flexible Publish plugin but did notice any change in behavior.