I have setup two fases:
build, containing the build jobs that make the binaries
test, containing the test jobs, the jobs testing the binaries produced by build jobs
The triggered jobs are listed only in the "Phases", while they are not listed in the "downstream builds" list. (see attachment)
Please note that the multijob is fingerprinting a file, containing univocal key (multijob name+build number, e.g. multijob-project#2), while the build jobs and the test jobs are also fingerprinting the same file (containing the triggering job and triggering build).
In jobs different from multijob projects this ensure the link from triggering job and downstream jobs (also rerun of a failed downstream job is linked in the triggering job)
I have also tried to trigger the test jobs in a post-build action, but also in this case the "downstream builds" list results empty.