1) Install the Join Plugin - https://wiki.jenkins-ci.org/display/JENKINS/Join+Plugin
2) Install the Downstream-Ext Plugin - https://wiki.jenkins-ci.org/display/JENKINS/Downstream-Ext+Plugin
3) Create 4 new projects (setup similar to Join Plugin example - test, testDown1, testDown2, testJoin) - see 4 XML files attached.
4) In the "test" project, when using "Build other projects (extended)" the "Downstream Projects" are listed as "testDown1", "testDown2" and "testJoin".
5) Configure the "test" project to use "Build other projects" instead and note how the "Downstream Projects" are now listed as "testDown1" and "testDown2".

It appears the "testJoin" project is incorrectly listed as a Downstream project when using this plugin.