Hot Water Surge Issue

Had a new Bradford White 50gal gas water heater installed last year (along with a recirc pump and related lines). We discovered a problem a few months later. If we run the shower (or tub faucet) for about 15 minutes, we get a HOT 20+ degree temp spike. We can't replicate at other faucets, just the shower/tub. The installer replaced the gas valve, but unfortunately it's still happening. I'm going to give them a call back of course. Just wondering if anyone has experience with this problem. I'm assuming it has something to do with the volume of water being used and reheating, but I'm just theorizing.