_wp_attachment_metadata.sizes is empty for REST API uploads, but populated correctly when uploading via wp-admin

[darrelsilveira047]

Environment
WordPress: 7.0.2
PHP: 8.3.29 (fpm-fcgi)
Server: Linux 6.6.139.1-1.azl3 x86_64
Web Server: nginx 1.28.0
Hosting: Azure App Service
Upload method under investigation: POST /wp/v2/media

Description
When uploading images through the REST API (POST /wp/v2/media), the image variants (thumbnail, medium, large, etc.) are successfully generated and written to both the local wp-content/uploads directory and Azure Blob Storage.
However, the attachment metadata returned by the REST API contains an empty sizes array:

"media_details": {
  "sizes": {}
}

The key point is that uploading the exact same image through the standard wp-admin Media Library uploader works as expected:
Image variants are generated.
_wp_attachment_metadata['sizes'] is correctly populated.
The REST API subsequently returns the expected media_details.sizes.

This suggests the issue is specific to the REST API upload path rather than image generation itself, as the same server, PHP configuration, image editor, and storage backend all work correctly through the admin uploader.

Steps to Reproduce

1. Upload an image using POST /wp/v2/media.
2. Inspect the response.
3. Observe that media_details.sizes is an empty object ({}).
4. Verify that the variant image files exist in both:

• wp-content/uploads
• Azure Blob Storage

5. Upload the same image through the wp-admin Media Library.
6. Observe that media_details.sizes is correctly populated.

Expected Behaviour
media_details.sizes should contain the generated image variants regardless of whether the image was uploaded through the REST API or via wp-admin.

Actual Behaviour
When uploaded via the REST API:
✅ Image variants are physically generated.
✅ Variant files exist locally and in Azure Blob Storage.
❌ _wp_attachment_metadata['sizes'] is empty.
❌ media_details.sizes in the REST response is {}.

When uploaded through wp-admin:
✅ Image variants are generated.
✅ _wp_attachment_metadata['sizes'] is populated correctly.
✅ media_details.sizes is returned as expected.

Question

Does anyone have any ideas what could be causing this behaviour?

When uploading via the REST API (POST /wp/v2/media), the image variants are physically generated and stored correctly, but _wp_attachment_metadata['sizes'] is empty. As a result, the REST response also returns an empty media_details.sizes, and even the top-level width and height values in media_details are empty.

Uploading the exact same image via wp-admin correctly populates all of this metadata.

Is there a known reason why the metadata would not be populated when uploading through the REST API, even though the variant images are successfully generated?