And does the heating stem from the wire-to-terminal interface from or the contact-to-blade interface?
My first instinct says it's the latter; inadequate contact area and pressure for the continuous current.
What is physically different about the receptacles that don't overheat? Or do we need a new configuration?
This is just my opinion. I don't work for a manufacturer nor have I take part in testing.
I believe it is the contact blades and the insulating materials. Cheaper plastics can melt slowly until eventual failure. It would be hard to tell by testing. Cheaper metal could cause a higher than expected temperature that the wire can handle but the plastics can not. There is also the possibility of the contact blades getting micro-scratches that negatively impact their ability to maintain good contact and eventually overheat/fail.
It could also be the chargers that plug into them. Like they are creating heat because those wires are sized to the minimum then the contact blades will of course get hotter and eventual failure occurs. Or partial connection where the charger is only partially inserted or where the charger's blades aren't the correct length, width, etc. to cover the surface area of the receptacle.
Lastly, the °C temp rating on the terminals where the branch circuit lands might not be straight forward and the average joe (homeowner, electrician, handyman, engineer, etc.) is using #8 when they should be using #6. That could create an issue with long term heating and cooling cycles but might not show up at the breaker since it is not exceeding the rating.
