Job groups with several threads may not return a current job

When several jobs are running in a job group (say, several PDF export jobs in XWiki), the job manager returns null as current job when any of the jobs ended even when other jobs in the group are still running. To reproduce, a way would be to start two long-running and then a short-running job in a group that has three threads (like PDF export by default). Then get the currently running job - it returns null even though two jobs are still running.