localization.fallback: true causes cross-locale draft leakage with autosave enabled

Describe the Bug

Description

When using localization with fallback: true together with drafts and autosave, values from the default locale are written into other locales after switching locale and triggering autosave.

This results in cross-locale data leakage between drafts.

Config

localization: {
locales: ['en', 'de'],
defaultLocale: 'en',
fallback: true,
},

versions: {
drafts: true,
},

admin: {
autoSave: true,
}

Expected behavior

With fallback: true, values from the default locale should only be used for read-time resolution.
They should not be written into other locales when saving drafts.

Actual behavior

The fallback value is returned by the API and then written back during autosave, effectively copying default locale content into other locales.

Notes

Setting fallback: false prevents this behavior.

This suggests that fallback resolution is applied before draft persistence and autosave writes back the resolved value instead of the raw stored value.

Link to the code that reproduces this issue

Reproduction Steps

Create a document in en

Enter a value in a localized field (e.g. title = "Hello")

Wait for autosave (draft created)

Switch locale to de

Localized fields appear empty in UI (expected)

Type a single character in de to trigger autosave

After autosave completes, the en value appears in de

Which area(s) are affected?

Not sure

Environment Info

Payload v3.74.0  
Next.js 15 (App Router)  
MongoDB adapter

No file, test, or code entry point is identified; start by reproducing the listed en/de locale switch and autosave sequence with fallback, drafts, and the MongoDB adapter. Compare the stored localized values before and after the de autosave, and consider the issue done when fallback values remain read-only and are not copied into another locale.