The RX 480 isn't measuring Junction Temperature., but instead is providing an Avg. Temperature across Multiple Sensors equally placed around the Die.
It really isn't unusual for most Microchips to be operating at 95 - 110c at the Hottest Parts., while the Avg. for the whole Die is sitting at 70 - 85c.
With this said., my Reference RX 5700 XT (with Blower Cooler) rarely hits a Junction Temp above 92c.
Silicon Quality, Case Cooling, Ambient and Location can all play a big role here... as can the Quality of the Carbon Sheet used in lieu of Thermal Paste.
If for example the sheet is a few nm thinner than it should be for good contact., well that's going to have a noticeable effect; which is why it's a little baffling that Thermal Paste wasn't used.
When I do maintenance, it's one of the first things I plan to do is replace the Carbon Sheet with Thermal Paste, like Kryonaut.. as that can have quite a noticeable impact on Junction and Avg. Temperatures.