potato, I posted in your other thread but is it possible your die isn't fully covered with paste? I had issues with a GPU where the die wasn't fully covered therefore the temperature seemed okay but I was getting artifacts so it was obviously not cooling evenly across the die's face. Repasting with more thermal paste resulted in similar temperatures (good temps) but no artifacts at all. Therefore, maybe part of your core is covered and the rest isn't transferring heat?
When I do bare die stuff now I just put a blob (Shin Etsu x23), the result is usually better than small grain of rice.
By blob I mean I use about 2-3x the amount I would normally instinctively use, so maybe 3 grains of rice.