Apprecciate the write out.
Apart from the paste (liquid metal instead of standard tim), sanding on the inside of the IHS and the shorter cure time that is pretty much my procedure. (usually 3 hours for LGA1151 and letting it dry overnight for LGA2066)
Isn't the first delid either my guess would be this is the 15th or something for LGA2066 and the only guy that had wierd temps was pumping 1,35Vcore through an 18 core paired with an AIO (Usually 15-20 degrees drop is what they reach easy compared to pre-delid)
For reference the other delid with this copper IHS on a 7920X did about 65 degrees tops in prime with +- 1,3Vcore in a normal sized custom loop. (1x360 iirc)
So yeah currently most parts have been checked and double checked.
Hopefully we can get a nice imprint of the die itself to check if the shape is that much off the standard. Hell at this point it may just be a faulty tube of conductonaut considering i'm running out of other ideas of what it could be.
Wel it seems an update is in order.:
After testing by rsnubje under the custom loop temps were actually good/okay ? (Based on 5x R15 loop, picture in OP was just a single run)
Pre-delid 4,7Ghz 1,25Vcore :
After delid 4,7Ghz 1,25Vcore:
Hell it can even do 4,8 on 1,3Vcore now :