#RequestHeader behaviour not as expected; working without required User-Agent - spring-boot

Question

I have a project set up using Spring Boot with Kotlin to make REST APIs.
I'm trying to use the #RequestHeader to recognize the User-Agent. The said header is required=true -
#PostMapping("details", produces = ["application/json"])
fun addInfo(#RequestHeader(name = "User-Agent", required = true) userAgent: String,
#Valid #RequestBody podEntity: PodEntity): ResponseEntity<String> {
pod.addPod(podcastEntity)
return ResponseEntity<String>("{ \"status\":\"Added\" }", HttpStatus.CREATED)
}
Problems -
However, even if I'm not sending the User-Agent header, the API is working and adding data. But, if I change the header to any other non default names, like, user or request-source, the required=true requirement is enforced to and the API does not work. Does it mean that default headers cannot be made mandatory using the required tag?
The other problem is that in the case of custom header, when the required header is missing for the request, the API fails by giving 400 error code but does not throw any exception. I was expecting HttpClientErrorException for my junit test case but on checking the console, I see no exception. Adding #Throws is also not helping. How do enable my function to throw an exception when the required header is missing?
Unit test -
#Test
fun test_getAll_fail_missingHeaders() {
val url = getRootUrl() + "/details/all"
val headers = HttpHeaders()
val request = HttpEntity(pod, headers)
try {
restTemplate!!.postForEntity(url, request, String::class.java)
fail()
} catch (ex: HttpClientErrorException) {
assertEquals(400, ex.rawStatusCode);
assertEquals(true, ex.responseBodyAsString.contains("Missing request header"))
}
}