After upgrading the Join plugin to 1.9, it does not work if one of the downstream tasks is a matrix job. This is true for the basic join functionality, as well as the 'Run post-build actions at join -> Trigger parameterized build on other projects' functionality. Downgrading the plugin to 1.8 fixes this problem (of course, 'Run post-build actions at join' is broken with 1.8).

Here's a simplified repro, where the 'downstream tasks' consists of a single matrix job:

• Create Job_A (free-style). Make it call Job_B with 'trigger parametrized build', and set up some dummy pre-defined parameters to send to Job_B. Also make it call Job_C as the 'join' job, in the 'Projects to build once, after all downstream projects have finished' field
• Create Job_B (matrix), give it some axis and values
• Create Job_C (free-style)

With that configuration, if I run Job_A, Job_B will run, but Job_C will not run. I have tested this with a mix of freestyle and matrix downstream jobs, with the same results - all the downstream jobs run, but the 'join job' does not run. I see the same behavior if I put Job_C into the Run post-build actions at join -> Trigger parameterized build on other projects' field instead of the 'Projects to build once, after all downstream projects have finished' field.

If I downgrade the Join plugin to 1.8, this test works.