system
A chat on subreddit /r/pcmasterrace
human
That seems to prove that they didn't even bother QC testing the cooling solutions before using them. What the fuck?
gpt
The heatsinks were outsourced, probably to PC Partner (the company owning Zotac and a few others, known also for making most AMD reference cards, and most sapphire cards). 

AMD probably requested prototypes, tested them and they were fine. They then ordered a large batch, tested a few random picks, and those happened to be fine as well. Then sold them all, and realized not all are working well.

Either bad batches, or the heasink manufacturer decided to cheap out half way into the production, or the vapor chamber is designed that close to the edge that random production variations are enough to result in some dud coolers.

Either of these is incredibly hard to catch with QC, even more so if you are not the company manufacturing the coolers yourself.