I'm running a 2700x with the stock Prism cooler and a new MSI 5700 XT Gaming X inside a Corsair SPEC-02 Redshift mid tower case and I'm doing ok with cooling on the Medium Folding Power setting. There are times the CPU hits about 80.3C but otherwise normally stays in the mid-70s for the most part. That said, there have been times where I've hit the Pause button to let the Prism catch up a bit. A realize that a 3900x might run a little hotter than a 2700x, but my mitigation was keeping it on Medium since, like you, High made the CPU temps rise far more quickly. You certainly may have done this already, but just my attempt to give the Captain Obvious take on it.
A thought...what kind of thermal paste is the cooler using? Mine is the stock also, but just another thought...
Even when I was running it with the RX 580, I never noticed it failing. Interesting.