[Bug]: net.fetch() fails to forward html form submissions intercepted with protocol.handle()

[BurningEnlightenment]

Preflight Checklist

• I have read the Contributing Guidelines for this project.
• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a bug report that matches the one I want to file, without success.

Electron Version

25.9.8, 26.6.2, 28.0.0, 29.0.0-alpha.3

What operating system are you using?

Windows

Operating System Version

Windows 11 Enterprise 22H2 22621.2715

What arch are you using?

x64

Last Known Working Electron version

None--it doesn't work with any electron version shipping protocol.handle().

Expected Behavior

It is documented that if one intercepts a protocol (e.g. given s: Electron.Session s.protocol.handle('http', ...)) one can forward a request to the built-in handler by passing bypassCustomProtocolHandlers: true in the RequestInit parameter.

Actual Behavior

However, if the request originates from a plain html <form> submission, the built-in handler fails with

Error: net::ERR_FAILED
at SimpleURLLoaderWrapper.<anonymous> (node:electron/js2c/browser_init:2:49030)
at SimpleURLLoaderWrapper.emit (node:events:517:28)
(node:9824) electron: Failed to load URL: http://127.0.0.1:3000/ with error: ERR_UNEXPECTED

I observed that the form submission request contains an origin: null header and that removing said header from the request allows the request to succeed. The request also succeeds if no custom protocol handler is installed.

Testcase Gist URL

https://gist.github.com/BurningEnlightenment/83ef1268fa71ebe0fafd9dfa4cc2811f

Additional Information

The Testcase contains an html form which can be submitted to the included koa webserver.

[ckerr]

CC @nornagon