SPFx web part pages stopped working under lists

Target SharePoint environment

SharePoint Online

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

💥 SharePoint Framework

Developer environment

None

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

• All latest Edge/Chrome
• SPFx version: 1.18.2 (tested multiple builds)
• Node.js version: 18.19.1

Describe the bug / error

Using the publishing or UI to create a single webpart SPFx page, that loads an SPFx web part.
We then copy the page into a list to be able to use it as a full-page modern form.

This page used to work with no problems before last Friday, April 25 2025.

Today these pages get redirected to an error page:

Ref support ticket:
https://answers.microsoft.com/en-us/msoffice/forum/all/error-message-error-error-initializing-application/62e389d3-bbae-4fad-8de5-fda0ba31961a?rtAction=1745870258001

It seems the SPFx framework does not load web parts into those pages anymore.

This breaking change was released directly to production, without going through to first-release as we have nightly automated QA testing on a first-release tenant and it did not flag the errors.

For context, we cannot use the custom form page directly from site pages library because if you set it as the list's form it will pass in an ID=X query string parameter, which will break the page if site pages does not have an item with the same ID.

This change essentially left us without any way to create a custom list form page and use it.

Steps to reproduce

1. create an SPFx web part
2. create a single page web part page under site pages that uses that webpart
3. create a list called "test"
4. copy the page from site pages into the "/sites/lists/test/test-spfx.aspx"
5. visit the page

Expected behavior

Web part should load properly.
Instead it crashes.