Spring: ConstraintValidator does not seem to be firing

Question

I have written a custom validator that will check the values of a json property against the field values of the elastic search document POJO it maps to. For example:

{
    "detail": "fullName"
    "values": ["Stack Overflow"]
    "active": "false"
}

data class DetailsDocument(
    val fullName: String,
    val address: String,
    val phoneNumber: String,
    val emailAddress: String
)

So if JSON like:

{
    "detail": "maritalStatus",
    "values":["married"],
    "active": "true"
}

comes in, it'll fail validation as maritalStatus is not a field in DetailsDocument.

My validator annotation and implementation class looks like this:

import javax.validation.Constraint
import javax.validation.ConstraintValidator
import javax.validation.ConstraintValidatorContext
import javax.validation.ReportAsSingleViolation
import javax.validation.constraints.NotNull
import kotlin.reflect.KClass
import kotlin.reflect.full.declaredMemberProperties

@Constraint(validatedBy = [DetailsFieldValidatorImpl::class])
@Retention(AnnotationRetention.RUNTIME)
@Target(AnnotationTarget.FIELD)
@NotNull(message = "Value cannot be null")
@ReportAsSingleViolation
annotation class DetailsFieldValidator(val sourceClass: KClass<*>, val message: String = "Value is not valid")

class DetailsFieldValidatorImpl : ConstraintValidator<DetailsFieldValidator, String> {

    private var valueList: List<String>? = null

    override fun isValid(value: String?, context: ConstraintValidatorContext?): Boolean {
        return valueList?.contains(value?.toUpperCase()) ?: false
    }

    override fun initialize(constraintAnnotation: DetailsFieldValidator) {
        valueList = constraintAnnotation.sourceClass.declaredMemberProperties.map { it.toString().toUpperCase() }
    }
}

With my usage being:

data class Account(

    @DetailsFieldValidator(sourceClass = DetailsDocument::class)
    val name: String,
    val values: List<String>,
    val active: Boolean
)

However, when I send in the invalid json, it doesn't look like the validator kicks in and rejects the block. My request goes on through and fails at the expected point give the invalid value.

Am I missing adding my validator to some sort of registry or something? Maybe a missing dependency? I've tried adding:

implementation("javax.annotation:javax.annotation-api")
implementation("org.springframework.boot:spring-boot-starter-validation")

but changes nothing.

One very important note that I found; I read on one of the answers on here that Spring does not recognise annotations in Kotlin, but have been unable to find any documentation supporting this.

SpringBoot: v2.3.0.RELEASE
Kotlin: v1.3.72

EDIT

I have tried adding a Java implementation, but still same issue, so fairly confident this is not a Kotlin support issue.