Microsoft briefly mentions a simplified version of the problem in this documentary: https://www.youtube.com/watch?v=z2d6IMBS...

And here's some (very passionate) commentary over it: https://www.youtube.com/watch?v=9lW82Dhx...

The solder bumps between the die and substrate crack as mentioned in the two videos above, and the reason is that the underfill material that is supposed to support the bumps is defective, the Tg is too low. Basically, it liquifies and doesn't support the solder anymore, causing it to crack. It will always be defective in this way, and reflowing can really damage electronics if done too much, and especially if done wrong, like what the iFixit guides tell you to do. It will never be a fix. Bolt mods like the "Fix Kit" also break things, and are pointless because the X-Clamps are fine anyways, why would Microsoft use them on the Xbox One and onwards if they weren't? The only fix it to replace the GPU.

The underfill issue is touched on here (the PS3 had the same problem): https://www.youtube.com/watch?v=Za7WTNwA...