Just a note; we tried Gelid, Grizzly and other pastes from Mx2 to JunPus, and we would get the cracking issue every single time (we dont tight the screws to avoid bending the cpu) between -140 C and - 165 C as Phil wrote.
Glue-ing the IHS back to the PCB seems to help reducing the mounting pressure that used to go all through the paste - which should cause the cracking issue.