Jobs failing to queue on production Bamboo instances (5.7-m2)

Summary:
A build is run and all of the jobs in the first stage appear in the "pending" state. When pending job details are shown, Bamboo gives the following status:
Job has not yet been queued. Waiting for prior stages to complete

In every case, the agent matrix shows that suitable agents are available for the jobs.

On one of the servers, IBAC, the build queue was completely empty - there was 0 activity on the instance, yet the 'stuck' build wouldn't start.

We've had this happen 4 times in less than 24 hours on 2 different production Bamboo instances (IBAC and JBAC). More information is available on BUILDENG-6431

No log output was produced that mentioned the stuck job keys. DEBUG-level logging has been enabled on IBAC in case it happens again.