When using “Rebuild Maven repository metadata” task, maven-metadata.xml that represents plugin groups, has and extra “groupId” tag. As maven documentation states here (http://maven.apache.org/ref/3.5.0/maven-repository-metadata/repository-metadata.html) there are two types of matadata content: artifacts /artifact versions (groupId, artifactId, version, versioning) and group of plugins (plugins).
Metadata generated by Nexus 3
Metadata in Maven Central (http://repo1.maven.org/maven2/org/codehaus/mojo/maven-metadata.xml)
When using the maven repository directly that is not really an issue but when that repository participates in maven repository group is one.
Current Nexus 3 matadata merge (RepositoryMetadataMerger.merge) has a validation bug so when merging metadata for plugins from different repositories may result in a incomplete result.