Edit: Just upgrade to Spring Boot 2.5.1 where this is resolved.

Old answer below:

I managed to fix this for now by adding exclusions to my spring-boot-starter-security dependency which apparently defined this Kotlin dependency.

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-security</artifactId>
    <exclusions>
        <exclusion>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-stdlib-jdk8</artifactId>
        </exclusion>
        <exclusion>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-stdlib</artifactId>
        </exclusion>
        <exclusion>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-reflect</artifactId>
        </exclusion>
    </exclusions>
</dependency>

Add @RequestBody to your controller method as

@PostMapping("/login")
fun login (@RequestBody req : AuthorizationRequest): String {
   return "received auth request: ${req.username}"
}

Response is:

I had structure like:

abstract class AbstractController (...) {
    fun save(@RequestMapping entity: T) {
    (...)
}

And to enable it I used

class DeriveredController (...) {
    @PostMapping
    override fun save(entity: Derivered) {
    (...)
    }
}

Problem was with lack of @RequestMapping in derivered function.

It turns out that step number 2) actually solves my problem here. I need to register a JavaTime module for the ObjectMapper. I must have done something wrong to not make this work earlier.

For example :

 var mapper = ObjectMapper() 
 .registerModule(KotlinModule())
 .registerModule(JavaTimeModule())

 mapper.readValue<DateDto>(message)

Also, there is no need to include com.fasterxml.jackson.datatype:jackson-datatype-jsr310 for example. You just include jackson-module-kotlin like so :

    <dependency>
        <groupId>com.fasterxml.jackson.module</groupId>
        <artifactId>jackson-module-kotlin</artifactId>
    </dependency>

To use the ObjectMapper and the jackson-module-kotlin seems to be sufficient! However, I would love some good resources / books about serialization and Jackson. How it works under the hood, and how to relate to it in practice. It is a lot of fragmented information about it. I will edit this answer if I find something.

Based on the information you gave, the problem seems to be with using _ as JsonProperty value. You may have observed all your properties for MetaData contain _. Try removing all the underscores and see. I hope also you've registered the ObjectMapper, example in your Application class:

@Bean
open fun objectMapperBuilder(): Jackson2ObjectMapperBuilder
        = Jackson2ObjectMapperBuilder().modulesToInstall(KotlinModule())