Form Customizer - blank form is displayed with a console error

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

No response

Describe the bug / error

We have a FormCustomizer, but I don't know whether or not that is causing this issue.

1. Form is launched via this link. Notice it is in the QualityPlans site. https://contoso.sharepoint.com/sites/QualityPlans/_layouts/15/listform.aspx?PageType=4&ListId=20150891-6368-4565-b788-e7ce025c0fc8&ID=4850&RootFolder=*&env=WebViewList&loadFormWithNucleus=1

2. It redirects to the SPListForm.aspx page (I guess as expected):
   https://contoso.sharepoint.com/sites/QualityPlans/_layouts/15/SPListForm.aspx?PageType=4&List=20150891%2D6368%2D4565%2Db788%2De7ce025c0fc8&ID=4093&RootFolder=%2A&env=WebViewList&loadFormWithNucleus=1&Web=fdea8ca4%2Ddcd9%2D4a46%2Db97d%2Da64b990b7b54

3. An endless 'loading' icon is displayed in the middle of the page:

4. In the console logs, this error is shown:
   Error: Script error for /_layouts/15/2057/strings.js. Script error for "/_layouts/15/2057/strings.js" https://requirejs.org/docs/errors.html#scripterror

5. Note that the locale ID is 2057. This is correct and "English - UK" is the site language for both the /QualityPlans site and the root site.

6. The /_layouts/15/2057/strings.js script does not exist in the root site, but /_layouts/15/1033/strings.js does exist.

Steps to reproduce

We have not been able to reproduce this on any other site.

Expected behavior

Form should load without a failure to load the /scripts.js file.