Job A triggers a variable number of job B depending on a environment variable. It can be zero but since "Counter Parameter Factory" doesnt allow zero repetitions (Java exception) I had to include this step as a conditional build step. Job A must wait until the end of all B jobs so option "Block until the triggered projects finish their builds" is selected (because it is really a diamond execution).
This jobs configuration works fine but if you launch another A (in parallel) when it finish its first step and reach conditional step, it get stuck waiting until the end of all the jobs from first execution, once first A execution finished second execution continue executing its B jobs.
This is a kind of interlocking process between executions.