Resolution:
When there is only a single Engine instance present on a Broker, the scheduling system will no longer assign tasks proportional to their relative priorities. Instead, it behaves as if Serial Priority Execution, located on the Manager Configuration page in the Services section, is set to True.

This issue is triggered by the use of a single Engine, and is known to exist in all currently supported versions of GridServer. The scheduling system is designed to assign work efficiently and correctly for multiple Engines. On account of this, and since GridServer is intended for use in an environment with a large bank of compute resources, the modification required to address this issue would lead to a needless decrease in performance. There are currently no plans to modify the behavior of the scheduler in current or future versions of GridServer.