All a bit integrated, on the face of it Spirit work in Wichita, and Boeing in Renton, but it seems the product was shipped from Wichita to Renton, doubtless after inspection by Boeing, but in the Boeing plant after delivery there were Spirit employees working on it again. This is a classic for things to fall between the lines of responsibility.
Likewise it is not unknown for there to be complete QA checking, issues found, but the resulting fix is not then subject to the same rigorous QA that was originally done. especially where the schedule allows for a major QA check, but then a short time for any fixes, and doesn't schedule a complete check again.