If I had to guess, the copper nuts probably turned soft during high temps, lowering bolt preload leading to back-out. You might want to consider high temp stainless locknuts - the type that are deformed. Also, torque them to at 60 - 80% of the stud yield strength as a primary means of back-out prevention. Recommend lubricating them prior to installation to prevent galling, especially if you have stainless-on-stainless.