Duplicate Selected Locales: content is empty when the admin active locale is not among the selected locales (remaining case after #15849)

Describe the Bug

Follow-up to #15847 / #15849 (fixed in v3.83.0) and #15655. Those fixed the filtering so localized block content is no longer dropped. A related case remains on v3.86.0:

"Duplicate Selected Locales" produces a document with empty localized content whenever the admin's active locale is NOT one of the selected locales.

Root cause

In packages/payload/src/duplicateDocument/index.ts (getDuplicateDocumentData), the source doc is first correctly narrowed to the selected locales:

duplicatedFromDocWithLocales = filterDataToSelectedLocales({
  configBlockReferences: payload.config.blocks,
  docWithLocales: duplicatedFromDocWithLocales,
  fields: collectionConfig.fields,
  selectedLocales,
})

I verified this step is correct now — for selectedLocales: ['en'] it preserves content: { en: [...] } with all inner block values intact.

The problem is the next step, which resolves that already-filtered doc back to a single locale:

const duplicatedFromDoc = await afterRead({
  ...
  doc: deepCopyObjectSimple(duplicatedFromDocWithLocales),
  locale: req.locale,       // the admin's ACTIVE locale
  fallbackLocale: null,     // hardcoded — ignores the configured fallback
  ...
})

When req.locale (the admin's active locale) is not in selectedLocales, the with-locales doc no longer contains req.locale (it was just filtered out), and with fallbackLocale: null there is no fallback — so every localized field resolves to empty. The new document is created with empty content.

This is independent of field type; it affects any localized field and is fully determined by whether req.locale ∈ selectedLocales.

Link to the code that reproduces this issue

Same setup as #15847 — https://github.com/jakobpevec/payload-issue-repro — with one added step: change the admin's active locale to a locale you do not select.

Reproduction Steps

Collection with a localized blocks (or richText) field and ≥2 locales (e.g. en, de), and a document with content in both.

1. Open the document.
2. In the admin, switch the active locale (top-right locale selector) to de.
3. Open Duplicate → Duplicate Selected Locales and select only en.
4. Duplicate.

Actual: the duplicated document's localized content is empty.
Expected: the duplicated document contains the en content.

Control (proves the rule): with the active locale set to en (active locale ∈ selected), the same steps copy the content correctly. Verified across a full matrix — content is preserved iff the active locale is one of the selected locales:

active locale	selected	content copied
en	[en]	✅
de	[en]	❌ empty
de	[de]	✅
en	[de]	❌ empty
en	[en, de]	✅
de	[en, de]	✅
fr	[en, de]	❌ empty

Suggested fix

Resolve afterRead with a locale guaranteed to survive the filter — e.g. use req.locale when it is in selectedLocales, otherwise the first selected locale — and/or honor the configured fallback instead of hardcoding fallbackLocale: null.

Which area(s) are affected? (Select all that apply)

area: localization, area: admin

Environment Info

• Payload version: 3.86.0
• Database adapter: @payloadcms/db-postgres
• Node.js: 22

Start in packages/payload/src/duplicateDocument/index.ts at getDuplicateDocumentData and trace the filterDataToSelectedLocales and afterRead calls. Reproduce the issue with the linked setup by selecting only en while the active locale is de; done means Duplicate Selected Locales preserves the selected locale's localized content regardless of the active locale.