Face ID Security Flaw in My iPhone

Hello, my phone's Face ID is unlocking with my brother's face, even though it's not registered. We're not twins; there's a 4-year age gap between us, so according to their claims, it shouldn't unlock even if we were twins. I previously reported this to the Apple call center, but they couldn't find any solution. They suggested deleting and reinstalling Face ID, which I did, but unfortunately, it yielded no results. I still don't think my phone is secure.