Why does Springdoc not add "nullable" for Kotlin `String?`?

Question

Why doesn't Springdoc translate a Kotlin nullable attribute into OpenAPI nullable: true?

Example from their own unit tests (SpringDocApp9Test.kt):

class DemoRequest (

    @field:Schema
    val noRequiredNullableNoDefault: String?
)

becomes

{
  "openapi": "3.0.1",
  ...
    "noRequiredNullableNoDefault": {
            "type": "string"
          }, 

Although https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.1.md#schemaNullable knows "nullable" with the description "Allows sending a null value for the defined schema. Default value is false."