Why am I losing my DefaultRequestHeaders with DelegatingHandler?

I have a Named Client with the DefaultRequestHeaders being added at configuration level like so:

services.AddTransient<ValidateHeaderHandler>();
services.AddHttpClient("WeatherService", client =>
{
    client.BaseAddress = new Uri("http://localhost:abcde/");
    client.DefaultRequestHeaders.Add("Accept", "application/json");
    client.DefaultRequestHeaders.Add("Authorization", GetNewAccessToken());
})
    .AddPolicyHandler(retryPolicy)
    .AddHttpMessageHandler<ValidateHeaderHandler>();

Added to the client is a DelegatingHandler. As I step through the override in ValidateHeaderHandler, which inherits from DelegatingHandler:

protected override async Task<HttpResponseMessage> SendAsync(
    HttpRequestMessage request,
    CancellationToken cancellationToken)
{
    if (!request.Headers.Contains("Authorization"))
    { ... }
    return await base.SendAsync(request, cancellationToken);
}

The header will contain "Authorization" only on the first attempt of base.SendAsync(). After that execution, if Polly's retry policy is triggered, the ValidateHeaderHandler's override of SendAsync will execute again - as expected - but all header information is completely gone.