I've attached 2 images to this issue which demonstrates the incompatibility of the downstream-ext plugin and the join plugin.

If you refer to working.png, this is configured using the "Build other projects" option. As you can see from the timestamps and log messages the "testJoin" project clearly waits for the "testDown1" and "testDown2" projects to finish before being triggered.

If you refer to broken.png, this is configured using the "Build other projects (extended)" option. As you can see the timestamp when the "testJoin" project starts building is the same time as the "testDown1" and "testDown2" projects. There is no waiting, which is incorrect.

This issue is possibly related to https://issues.jenkins-ci.org/browse/JENKINS-9903.