Relations in localized fields (Advanced Many-to-Many / relations) are not persisted after saving in Studio UI

[vdxa-dev]

Description
Relations added to fields inside localizedfields (e.g. bodyStyle, manufacturer) are not saved correctly in Studio UI. After saving and reloading the object, the relation disappears. This makes localized relational data unreliable and unusable in Studio UI. This issue prevents one of our enterprise customers from effectively managing their data as they rely on Studio UI for several reasons.

Steps to Reproduce
(Observed on Pimcore Demo , April 14)

• Go to the Pimcore demo site: demo.pimcore.com :: Pimcore Studio
• Modify the Product Data / Car class: Move bodyStyle and manufacturer into localizedfields
• Save the class definition
• Open any DataObject of this class in Studio UI
• Add a relation to bodyStyle or manufacturer in any language
• Save the object
• Reload the object

Actual Behavior
The relation that was added is no longer present after reloading.

Expected Behavior
Relations added to bodyStyle or manufacturer inside localizedfields should persist after saving and reloading the object.

Affected Versions

• Observed on demo instance (likely latest dev version)
• Likely affects Studio UI versions similar to current demo

Environment

• OS: macOS 26.3
• OS: Windows 11
• Browser: Google Chrome Version 144.0.7559.133 (Official Build) (arm64)

[markus-moser]

Seems to be a backend bug, am able to reproduce it - in the classic UI I see the value in studio not. The strange thing is that with a new class definition it seems to work but with the described case it doesn't.