Was there ever a solution for this issue? I am seeing it right now.
27 clips on a reel, only 26 deliverables rendered. Cortex reports the reel as completed.
I am on version v3.0.3-b9978 (g17b015c).
Worst of all, when I re-opened the reel and closed it again, Cortex deleted all the previously-rendered files! Now, only the "missing" file is in the output directory, so the whole reel must be re-rendered.