No one can infer anything from 'local paste', '3 relatively high priced thermal paste', and 'currently using the best performing one'.
Thermal Grizzly's Kryonaut is hyped as one of the best pastes, but you know, it's not very good for gpu applications. It either gets pushed off the die, or degrades quickly to high temperature cycles.
Bare silicon dies have a slicker and slippery surface compared to the rough and grainy IHS of a cpu. These 3 pastes could be great on cpus, but terrible on gpus, because the paste either 'slips' off, or gets pumped out by mounting pressure and high thermal cycles.
Ideal pastes for gpu applications are ones with high(er) viscosity; thick and sticky.
I believe the sensor is working just fine.
-The paste being used.
-The heatsink's finstack having a horizontal design. I've no idea why it's not vertical, so as to 'guide' heated air out the sides. Instead, it's the front and back...
One or both of those 2. I'm going to accept that you've mounted the cooler correctly and not include it.