ASP.NET Core 8 TypedResults.File with ETag always returning result from browser disk cache

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I have a simple ASP.NET Core 8 endpoint that returns a TypedResults.File with an ETag. First time I hit the endpoint, the file is returned 100% ok.

Second (and subsequent) times I hit the endpoint with the same instance of the browser I get an HTTP 200 OK (from disk cache) – it never makes the actual request to my localhost server.

Here’s the network traffic of the 2x requests:

Request 1:

enter image description here

Request 2 (which grabs it from the disk cache)

enter image description here

Can anyone help me please? I thought that if the initial request returns an ETag then the browser is “smart enough” to do subsequent requests with the ETag + “If-None-Match” request header?

This is the code that generated the File

var etag = new EntityTagHeaderValue($""{invoicePdfVersion}-{lastModifiedOn.Ticks}"");

return TypedResults.File(
    pdfData, // byte array of pdf data 
    "application/pdf", // content type
    "testing", // filename
    false, // enable range processing
    lastModifiedOn, // DateTime when the file was last updated/modified
    etag); // etag value

I understand that I could add this to the response:

context.Response.Headers.CacheControl = "no-cache";

which I think does stop the disk cache occurring but i’m not sure if this is required? Shouldn’t the MS-Code for ReturnTypes.File set all the headers up correctly so I don’t need to do anything else?

I have some server side code which checks the ETag btw…

if (context.Request.Headers.TryGetValue(HeaderNames.IfNoneMatch, out var ifNoneMatch) &&
    ifNoneMatch == etag.ToString())
    return TypedResults.StatusCode(304); // HTTP NotModified.

but of course, this never gets hit.

Can anyone help?