See also this comment in the Unreal Engine code about putting a nop in before as well: https://github.com/EpicGames/UnrealEngine/blob/26677ca1b3c97...

    // Q: Why is there a __nop() before __debugbreak()?
    // A: VS' debug engine has a bug where it will silently swallow explicit
    // breakpoint interrupts when single-step debugging either line-by-line or
    // over call instructions. This can hide legitimate reasons to trap. Asserts
    // for example, which can appear as if the did not fire, leaving a programmer
    // unknowingly debugging an undefined process.

(This comment has been there for at least a couple of years, and I don't know if it still applies to the newest version of Visual Studio.)