I remember watching a video of someone writing C code and making the same thing in unsafe rust. While the C code worked just fine the rust code had UB in it and was compiled to a different set of instructions.

Unsafe rust expects you to uphold the same guarantees that normal rust does and so the compiler will make all the same optimisations it would if the code wasn't unsafe and this caused UB in the example rust code when optimised for performance. It worked just fine on the debug build, but that's UB for you.