Spring MongoDB - use abstract/sealed class field

Question

sealed class Entity

data class Bacteria(
    val uid: String,
    val rank: String,
    val division: String,
    val scientificname: String,
    val commonname: String
): Entity()

data class CTDDisease(
    val diseaseId: String,
    val name: String,
    val altDiseaseIds: List<String>,
    val parentIds: List<String>,
    val definition: String?
) : Entity()

And then I want to define my document as

@Document(collection = "annotations")
data class Annotation(
    @Id val id: String,
    ...
    val spans: List<AnnotationSpan>
)

data class AnnotationSpan(
    val startIndex: Int, 
    val endIndex: Int, 
    val entity: Entity? // Can be Bacteria or Disease or null
)

I also accept these 2 classes within the RequestBody from the client from time to time e.g.

@PutMapping("/annotations")
fun submitAnnotations(@RequestBody submission: Submissions): Mono<Void> { ... }

data class Submission(val annotations: List<AnnotationSpan>, ...) // AnnotationSpan contains Entity

but I get

java.lang.IllegalAccessException: class kotlin.reflect.jvm.internal.calls.CallerImpl$Constructor cannot access a member of class com.package.name.Entity with modifiers "private"

If I change the class Entity to an abstract class

abstract class Entity

then I don't get an error but my query operations keep going on forever.

Bacteria and Disease both have different fields so they should be distinguishable.

I tried using a hacky converter

@ReadingConverter
class NormalizedEntityReaderConverter: Converter<DBObject, NormalizedEntity> {
    override fun convert(source: DBObject): NormalizedEntity? {
        println(source.toString())
        val gson = Gson()
        return gson.runCatching { fromJson(source.toString(), CTDDisease::class.java) }.getOrNull()
                ?: gson.runCatching { fromJson(source.toString(), Bacteria::class.java) }.getOrNull()
    }

}

and then registering it like

@Configuration
class MongoConverterConfig {

    @Bean
    fun customConversions(): MongoCustomConversions {
        val normalizedEntityReaderConverter = NormalizedEntityReaderConverter()
        val converterList = listOf<Converter<*, *>>(normalizedEntityReaderConverter)
        return MongoCustomConversions(converterList)
    }
}

My converter seems to work when called manually but for some reason, Spring still isn't picking it up.

I'm new to Spring. I would achieve this functionality in my Node.js server by using union types in TypeScript e.g.

interface AnnotationSpan {
    startIndex: number;
    endIndex: number;
    entity?: Bacteria | Disease;
}

How can I achieve this behavior?