serializedValue silently loses nullable fields in non-nullable nested objects during read-modify-write

[RiteshChandnani]

Library version: 1.3.0
Kotlin: 2.1.0
Platform: Android

---

Description

When using serializedValue with a @serializable class that contains a non-nullable nested object (with a default value) whose fields are all nullable, a read-modify-write cycle silently loses all written values. The write appears to succeed (no exception), but the subsequent read returns the default (null) for every field inside the nested object.

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Minimal Reproducible Example

 @Serializable
 data class Document(
     val title: String? = null,
     val metadata: Metadata = Metadata(),
 )
 
 @Serializable
 data class Metadata(
     val author: String? = null,
     val version: Int? = null,
 )
 
 @OptIn(ExperimentalSettingsApi::class, ExperimentalSerializationApi::class)
 class Storage(settings: Settings) {
     var document: Document by settings.serializedValue(
         serializer = Document.serializer(),
         key = "document",
         defaultValue = Document(),
     )
 }
 
 @Test
 fun `nested nullable fields survive read-modify-write`() {
     val settings = Settings()
     val storage = Storage(settings)
 
     // Read-modify-write — update only a field inside the nested object
     storage.document = storage.document.copy(
         metadata = storage.document.metadata.copy(author = "Alice")
     )
 
     // FAILS: returns null instead of "Alice"
     assertEquals("Alice", storage.document.metadata.author)
 }

---

Expected behaviour
storage.document.metadata.author returns "Alice" after the write.

Actual behaviour
storage.document.metadata.author returns null. The write appears to succeed silently but the value is not recovered on the subsequent read.