GitLab pipelines for merge requests in Jenkins - jenkins

Question

I'm working on a CI integration between GitLab and Jenkins where I want to do specific validation depending on the target branch of an opened merge request (MR).
To be more specific, we have a repository containing Flyway database migrations and we'd like to validate inside a Docker container whether the migrations can be applied successfully against different environments. The target environment can be deduced by the target branch of a merge request.
Now, my problem is this. I'd like to reject MRs that don't build successfully, but I couldn't find way of rejecting MRs, only commits.
The way our integration is set up is that Jenkins receives webhook notifications for both commit and MR events. So, when someone pushes a new commit to an MR, two Jenkins builds will be triggered — one for the whole MR and one for the commit. The MR build will fail, but the commit build will succeed and due to race conditions in how the builds are run, the MR pipeline in GitLab will be marked as successful although it shouldn't be.
Is there a way to mark merge requests as failed in a Jenkins-GitLab integration?
I believe this feature is called pipelines for merge requests in GitLab's built-in CI/CD support, but can it be simulated with Jenkins?
Here's a sketch of the Jenkinsfile I'm using:
def isMergeRequest() {
gitlabActionType == 'MERGE' && gitlabTargetBranch != gitlabSourceBranch
}
pipeline {
agent any
stages {
stage('Validate') {
when {
expression {
isMergeRequest()
}
}
steps {
gitlabCommitStatus('Validate') {
sh "jenkins/validate_mr.py $gitlabTargetBranch $gitlabSourceBranch"
}
}
}
}
}