yeah this is an annoying phenomenon. When using garage cars as A.I. in custom race, I'm not 100% sure if the A.I. uses a stock sheet setting, but your garage cars will revert to the stock default tyres they're fitted with upon purchase.
Edit: I concur, A.I. cars will only revert to stock tyres if they're fitted with any tyre grade below hard slicks.
It's got to be a bug, would like to see this addressed.