🐞 SPFx Field & ListForm Customizers not loading from local dev - SPO reporting CORS issues even after preloading

Target SharePoint environment

SharePoint Online

What SharePoint development model, framework, SDK or API is this about?

💥 SharePoint Framework

Developer environment

macOS

What browser(s) / client(s) have you tested

• 💥 Internet Explorer
• 💥 Microsoft Edge
• 💥 Google Chrome
• 💥 FireFox
• 💥 Safari
• mobile (iOS/iPadOS)
• mobile (Android)
• not applicable
• other (enter in the "Additional environment details" area below)

Additional environment details

• browser v143.0.7499.194
• SPFx v1.20.0 & v1.22.1
• Node.js v18 & v22

Describe the bug / error

When running field customizer & list form customizer projects from two different versions of SPFx (v1.20 & v1.22.1), lists are loading the locally hosted manifest.js and bundle, but the bundles aren't being executed (even a simple console.log() within lists).

To be clear: other non-field customizer SPFx projects work fine (web parts, app customizers, command set extensions, etc).

Both projects have worked in the past on the same list. This also happens with a brand new SPFx v1.22.1 field customizer project with zero code changes from the default template.

Upon further inspection, it appears the SharePoint list infrastructure is failing with a preload / CORS issue. The following errors are showing in the browser's console, and in the local console, showing SPO is requesting and loading the files.

For the SPFx v1.20.1 project

For the SPFx v1.22.1 project

Steps to reproduce

1. Create a new SPFx v1.22.1 field customizer extension
2. Execute heft start --nobrowser
3. Open a list in SPO and add the querystring to force the field customizer to load
4. Observe the above issues

Expected behavior

Um... it works...