Steps to reproduce:

1. Add any valid _middleware.ts to a NextJS application.
2. Deploy to Vercel
3. Make a POST request to any url where that middleware will be run with: Expect: 100-continue as an HTTP header, and any non-empty body content.

Result:

We get a 502 Bad Gateway with the following response body:

An internal error occurred with Vercel.

INTERNAL_EDGE_FUNCTION_CONNECTION_CLOSED

In the function log, we get a message that the middleware was called, but the code does not execute (adding a console.info in the middleware does not show up in the function log)

Expected result:

Middleware and page or api executes appropriately.

Expect: 100-continue is part of the HTTP 1.1 spec and should be implemented, or should be documented explicitly as not implemented.