SQLSTATE[23000]: Integrity constraint violation
When setting up this product variation, I selected for the SKU to be automatically generated. In doing so, it was tripping up the feeds importer. Not sure if this was a timing issue, but I reviewed the importer set up and product variation fields for the importers that were working vs. not.

This SKU setting was the only difference. Since this field is unique ID, this seemed like the most likely culprit. 

I would appreciate an explanation of why this was triggering an issue, but am happy to be able to move on.