Content Type changes are ignored for an uploaded Office document when Form Customizer is enabled for a Content Type

Target SharePoint environment

SharePoint Online

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

💥 SharePoint Framework

Developer environment

Windows

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

Chrome version: 145.0.7632.117
SPFx version: 1.22.2
Node.js version: 22.17.0

Describe the bug / error

SharePoint does not change the content type of Office documents (e.g. Excel) uploaded to a document library that uses custom content types with form customizer. Both the UI and the REST API fail to update the content type.

In the content type details panel, content type change is reverted by SharePoint. When using the REST API from a form customizer (e.g. validateUpdateListItem), the request succeeds but the content type stays the same. It only fails for Office files when uploaded.

{ "d": { "ValidateUpdateListItem": { "__metadata": { "type": "Collection(SP.ListItemFormUpdateValue)" }, "results": [ { "ErrorCode": 0, "ErrorMessage": null, "FieldName": "ContentTypeId", "FieldValue": "0x01010070EA720D465B4CACA9BD902EEBCE95B200484D6EBC4F74744FB2AF64BB51C83A19", "HasException": false, "ItemId": 741 } ] } }

Steps to reproduce

1. Create a document library.
2. Add custom content types to the library.
3. Add form customizer to those content types.
4. Upload an Office document (e.g. Excel) to the library.
5. Try to change the content type in the details panel - SharePoint reverts the change.
6. Try to change the content type via REST API (validateUpdateListItem) from the form customizer - the request succeeds but the content type does not change.

Expected behavior

• The content type can be changed in the details panel.
• The content type can be updated via REST API (e.g. validateUpdateListItem) from a form customizer.