Given a project foo based on a maven multi-module approach with a groupid/artifactid/version (GAV) : foo:par:1.0.0-SNAPSHOT and ** with (at least) one submodule : 

• foo:elem1:1.0.0-SNAPSHOT

And a project bar based also on a maven multi-module approach with a GAV : bar:par:1.0.0-SNAPSHOT and ** with (at least) one submodule : 

• bar:elem1:1.0.0-SNAPSHOT that depends on foo:elem1:1.0.0-SNAPSHOT

Two Multibranch pipeline jobs based on each of this projects based one the same Jenkinsfile with such step :

withMaven(maven: 'maven-3.5.2', options: [pipelineGraphPublisher(includeReleaseVersions: true)]) {
  sh "mvn clean deploy"
}

If we're triggering a run job for project foo it won't trigger the build of bar project 

Additionally:

In the bar project the Maven "Downstream Jobs" section is empty 

In the bar project the Maven "Upstream Builds" section is empty 

Thanks a lot for your help or any insight !