It seems as though my own meddling may have detached the form from the workflow. I admit, I was poking around in InfoPath and could have saved/published a change I didn't understand. This resulted in the form being de-linked from the Workflow itself.

The solution was to recreate the workflow under a different name, remove the old one, and publish again. The result is that changes to the Initiation Parameters are now seen in the initiation form.