Seems like the game is referencing one set of default values when you enter the track (values for tyre compound, diameter etc) and another (screwed up set of erroneous data) when performing a pit stop. Whenever you go to track the tyre values seem correct lobby (but leave your car entered or 'on track'), when you go back to the car the tyre values are correct again and the bug is gone.
That sounds like the idea I had, that the wrong tire variant is being placed on the cars after a pit.
The reason is that there's no reason why, if you have the "correct" tires upon race start, that it couldn't simply copy what it did at race start, for the pit-stop tire-exchange. Because it isnt' happening, it must be placing something different on the cars. The "physics" don't change when the pit stop occurs. T here's no reason why they would code it that way. But perhaps the wrong reference is hidden in there somewhere, referencing the wrong tire.
The fact that the Comfort Soft tires did not seem to cause additional wear when swapped at a pit, makes me wonder if there is only ONE type of Comfort Soft tire in the game's code, and many types of Sports and Racing tires. We know from the GT5 cracking jobs people did, that there are different variants of tires. So, maybe someone could figure out if there's only one type of CS?
Maybe we should post our tests over there in the GTP thread? I finally signed up there to post some pics recently...