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

[andrewconnell]

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 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...

[Ashlesha-MSFT]

Hello @andrewconnell,
Thank you for bringing this issue to our attention. We will look into it and get back to you shortly.
If you don’t receive a reply within two working days, please use this escalation form to escalate.

[Ashlesha-MSFT]

In our case:
The extension works perfectly when deployed to production via App Catalog, but fails to execute in development mode despite the manifest and bundle loading successfully.
Tried the engineering team's workaround (&allowSpfxDevFallback=true) along with hard refreshes and &sw=bypass, but none work on our tenant. The field customizer's onInit() never fires, and we see the same CORS preload warnings described here.
The issue blocks the entire local development workflow for field customizers(Used - SPFx 1.22.1).

I was able to reproduce this issue consistently and will create a detailed bug report with our specific environment details.
Thanks!