Mailer CRON job limit does not work

There is a Mailer Setting for: Mailer CRON job limit ($config->mailerJobsMax)

This is supposed to limit the number of concurrent mailing processes. However the limit is ignored, and each time the process_mailing job is called a new mailing job is processed. Ie:

First cron job is called, acquires lock 1, starts sending a mailing job
Second cron job is called, acquires lock 1 again, starts sending a second mailing job

The code that is supposed to limit this is \CRM\Mailing\BAO\Mailing.php > processQueue > ~ln 2909

The cause

The script goes on to acquire a second lock, for the individual mailing job to process (in runJobs_pre). This releases the original cron limit lock. As described in CRM-12856 and CRM-17846