I have a register() function which makes a fetch() call to a Route Handler inside of a timeout like so:

export async function register() {
  if (typeof window === 'undefined') {
    setTimeout(async () => {
      try {
        await fetch('http://localhost:3000/api/boot', {
          method: 'POST',
        });
      } catch (error) {
        console.error(
          `Instrumentation failed for runtime: ${process.env.NEXT_RUNTIME}`,
          error
        );
      }
    }, 500);
  }
}

I have my instrumentation.ts file placed inside the /src directory, and I've enabled the experimental.instrumentationHook flag in my next.config.mjs file.

When I develop locally (next dev), the fetch call is made successfully, and if I make a console.log statement within register(), I'll see it in my terminal's output. However, once I deploy to Vercel, even though it builds correctly and the page loads successfully, there's no sign that the register() function is ever even called. No logs from within the register() function ever show up, and my Route Handler is never called.

I've been following this GH issue around instrumentation, but it's gone somewhat stale: https://github.com/vercel/next.js/discussions/15341#discussioncomment-5845714