SpringBoot custom ConstraintValidator not getting triggered - spring-boot

Question

My SpringBoot app has the following OpenAPI generated code.
In my RestController I have a custom ConstraintValidator #ValidIndexName on the indexName path parameter.
Why does the ConstraintValidator IndexNameValidator code below not get called?
I have the spring-boot-starter-validation dependency in my pom.
#javax.annotation.Generated(value = "org.openapitools.codegen.languages.SpringCodegen", date = "2020-03-23T16:08:36.286420Z[Europe/London]")
#Validated
#Api(value = "Document", description = "the Document API")
public interface DocumentApi {
#ApiOperation(value = "", nickname = "createDocument", notes = "", tags={ "Document", })
#ApiResponses(value = {
#ApiResponse(code = 202, message = "Accepted") })
#RequestMapping(value = "/docs/{indexName}/",
consumes = { "application/json" },
method = RequestMethod.POST)
ResponseEntity<Void> _createDocument(#ApiParam(value = "Elasticsearch index name",required=true) #PathVariable("indexName") String indexName,#ApiParam(value = "" ) #Valid #RequestBody Document document);
#ApiOperation(value = "", nickname = "updateDocument", notes = "", tags={ "Document", })
#ApiResponses(value = {
#ApiResponse(code = 202, message = "Accepted") })
#RequestMapping(value = "/docs/{indexName}/",
consumes = { "application/json" },
method = RequestMethod.PUT)
ResponseEntity<Void> _updateDocument(#ApiParam(value = "Elasticsearch index name",required=true) #PathVariable("indexName") String indexName,#ApiParam(value = "" ) #Valid #RequestBody Document document);
}
#RestController
#Validated
public class DocumentApiImpl implements DocumentApi {
#Override
public ResponseEntity<Void> _createDocument(String indexName, Document document) {
return createDocument(indexName, document);
}
public ResponseEntity<Void> createDocument(#ValidIndexName String indexName, Document document) {
System.out.println("indexName = " + indexName);
return new ResponseEntity<>(HttpStatus.ACCEPTED);
}
#Override
public ResponseEntity<Void> _updateDocument(String indexName, Document document) {
return createDocument(indexName, document);
}
public ResponseEntity<Void> updateDocument(#ValidIndexName String indexName, Document document) {
System.out.println("indexName = " + indexName);
return new ResponseEntity<>(HttpStatus.ACCEPTED);
}
}
#Target( { PARAMETER })
#Retention(RUNTIME)
#Constraint(validatedBy = IndexNameValidator.class)
#Documented
public #interface ValidIndexName {
String message() default "TenantId cannot be used as an index name.";
Class<?>[] groups() default {};
Class<? extends Payload>[] payload() default {};
}
#Component
public class IndexNameValidator implements ConstraintValidator<ValidIndexName, String> {
#Override
public boolean isValid(String value, ConstraintValidatorContext context) {
System.out.println("Validating value = " + value);
if (StringUtils.isEmpty(value)) {
return false;
}
return true;
}
}