When Polylang Pro plugin's Media translation is enabled and user uploads an image to Media gallery, the _wp_attachment_metadata does not include image sizes for the image for other than the default language.

Here is screenshot from database for uploaded image that has correct metadata (image sizes) for default language and incorrect meta for other language.

Disabling WP Stateless and uploading an image with same settings will result in correct metadata for both languages:

If thumbnails are regenerated and synced, the metadata for other languages are corrected.

Used WP Stateless 2.2.6 and Polylang Pro 2.5.3 for testing.

This I reopened this issue as it has not been corrected. Old issue is #306