Why isn't Swagger detecting optional JSON properties?

I have the following class:

import com.fasterxml.jackson.annotation.JsonProperty
import org.joda.time.DateTime
import org.joda.time.DateTimeZone

data class Entity(
        val email: String,
        val name: String,
        val birthDate: DateTime,
        @JsonProperty(required = false) val gender: Gender? = null,
        @JsonProperty(required = false) val country: String? = null,
        val locale: String,
        val disabled: Boolean = false,
        @JsonProperty(required = false) val createdAt: DateTime = DateTime(DateTimeZone.UTC),
        val role: Role,
        val entityTypeId: Long,
        val entityTypeAttributes: MutableMap<String, Any> = HashMap(),
        val medicalSpecialityId: Long? = null,
        val id: Long? = null
)

And some properties are not required, because they are either nullable (gender, country), or they have a default value (createdAt).

However, the generated swagger documentation is as follows:

 "components": {
    "schemas": {
      "Entity": {
        "required": [
          "birthDate",
          "createdAt", <------------ Notice here!
          "disabled",
          "email",
          "entityTypeAttributes",
          "entityTypeId",
          "locale",
          "name",
          "role"
        ],
        "type": "object",
        "properties": {
          "email": {
            "type": "string"
          },
          "name": {
            "type": "string"
          },
          "birthDate": {
            "type": "string",
            "format": "date-time"
          },
          "gender": {
            "type": "string",
            "enum": [
              "MALE",
              "FEMALE",
              "OTHER"
            ]
          },
          "country": {
            "type": "string"
          },
          "locale": {
            "type": "string"
          },
          "disabled": {
            "type": "boolean"
          },
          "createdAt": {
            "type": "string",
            "format": "date-time"
          },
          "role": {
            "type": "string",
            "enum": [
              "ADMIN",
              "DOCTOR",
              "PATIENT"
            ]
          },
          "entityTypeId": {
            "type": "integer",
            "format": "int64"
          },
          "entityTypeAttributes": {
            "type": "object",
            "additionalProperties": {
              "type": "object"
            }
          },
          "medicalSpecialityId": {
            "type": "integer",
            "format": "int64"
          },
          "id": {
            "type": "integer",
            "format": "int64"
          }
        }
      },
   (...)

So in terms of documentation it shows that createdAt is mandatory (which is not true)...

Generated Swagger docs

I am using Kotlin, Javalin and the OpenAPI (io.javalin.plugin.openapi) Javalin integration.

I don't know what more do I need to make OpenAPI understand that createdAt is optional...