Jenkins restrict Jenkinsfile modifications on pull request
As far as I know it is not documented, but only a pull request from a branch of the repository can execute a changed JenkinsFile.If someone make a fork and a pull request, the Jenkins file exectued by Jenkins will be the one of the targeted branch of the pull request, and not the one of the pull request.
If admins level is required to merge anything (not only the master branch) so you are safe.
you probably forgot to add the handler for the jenkins file, look at the repository to get the setting of the preventation. Hope this will fix it for you.