Validation messages don't show in Thymeleaf - kotlin - spring boot - spring-boot

Question

I return the template of a form with this controller suspend method:
#GetMapping("/checkout/customer")
suspend fun checkoutCustomerView(model: Model): String {
model["cart"] = shoppingCartService.getShoppingCartWithItems()
model["checkout"] = checkoutService.getCheckout()
return "checkout/checkoutCustomer"
}
I post form data with:
#PostMapping("/checkout/customer")
suspend fun updateCustomerData(
request: ServerHttpRequest,
response: ServerHttpResponse,
model: Model,
#Valid #ModelAttribute customerData: CheckoutCustomerData,
bindingResult: BindingResult
): String {
if (bindingResult.hasErrors()) {
model["cart"] = shoppingCartService.getShoppingCartWithItems()
model["checkout"] = checkoutService.getCheckout()
return "checkout/checkoutCustomer"
}
model["checkout"] = checkoutWizardCookieService.updateCustomerData(customerData, request, response)
return "redirect:/checkout/shipping"
}
the binding gets populated when there's an error, I can see it in debug mode.
Data is:
data class CheckoutCustomerData(
#field:NotBlank
val firstName: String?,
)
and html file is:
<form id="customer-form" th:action="#{/checkout/customer}" th:object="${checkout}"
method="post">
<div class="mb-3 d-flex flex-row">
<input type="text" class="form-control me-3" th:field="*{firstName}" placeholder="First Name">
<p th:if="${#fields.hasErrors('firstName')}"
th:errorclass="invalid-feedback"
th:errors="*{firstName}" >The first name is mandatory</p>
</div>
</form>
when I submit the form without a value for the field "firstName", I should expect some error showing under the input field, which doesn't happen, because the form is considered valid, so #fields.hasErrors() returns false.
I am not sure what I am doing wrong.
I am using Spring Boot Webflux with kotlin and coroutines. Is there some extra configuration due to the reactive nature of the project, maybe?
Thanks
Is