The attached jar files have valid manifests:

multirelease-0.8-SNAPSHOT_failure.jar
-----------------------------------------------------
Manifest-Version: 1.0
Archiver-Version: Plexus Archiver
Created-By: Apache Maven 3.3.9
Built-By: scholterf
Build-Jdk: 9-ea
Multi-Release: true

multirelease-0.8-SNAPSHOT_success.jar
--------------------------------------------------------
Manifest-Version: 1.0
Archiver-Version: Plexus Archiver
Created-By: Apache Maven 3.4.0-SNAPSHOT
Built-By: scholterf
Build-Jdk: 9-ea
Multi-Release: true

Yet, JarFile thinks the "failure" jar does not have the "Multi-Release: true" attribute, demonstrated by

jshell> /op script
multirelease-0.8-SNAPSHOT_success.jar is multi-release? true
multirelease-0.8-SNAPSHOT_failure.jar is multi-release? false

jshell> /li

   1 : import java.util.jar.*;
   2 : File f = new File("multirelease-0.8-SNAPSHOT_success.jar");
   3 : JarFile jf = new JarFile(f, false, 1, JarFile.runtimeVersion());
   4 : System.out.println(f + " is multi-release? " + jf.isMultiRelease());
   5 : f = new File("multirelease-0.8-SNAPSHOT_failure.jar");
   6 : jf = new JarFile(f, false, 1, JarFile.runtimeVersion());
   7 : System.out.println(f + " is multi-release? " + jf.isMultiRelease());

Further investigation shows that JarFile::match returns -1 for the "failure" jar.