rails turbo frame apparently updating but browser window remains unaltered

The following _receipt_number partial (HTML formatting tags removed for brevity's sake)

<%= turbo_frame_tag :receipt_number do %>
      <%= t('scan') %> <%= t('cart.receipt_number') %>
      <h3><%= @cart.receipt_number %></h3>
      <% if @cart.receipt_number %>
        <%= button_to t('change'), clear_receipt_number_cart_path(id: @cart.id), class: 'button warning', method: :patch %>       
      <% end %>
<% end %>

is rendered in HTML as:

<turbo-frame id="receipt_number">
      scan receipt number
      <h3></h3>
</turbo-frame>

Via a javascript that controls the device camera a scan is run and is processed (this is the relevant javascript code for the instance)

let formData = new FormData();
let CodeParams = {
  code_data: result.text,
  id: <%= @cart.id %>
};
formData.append("code_json_data", JSON.stringify(CodeParams));
fetch("update_receipt_number", {
  method: "post", 
  format: "turbo_stream",
  body: formData,
  headers: {
    "X-CSRF-Token": document.querySelector("[name='csrf-token']").content,
  }
});
codeReader.reset();

via the controller action with:

@cart.update(receipt_number: result['code_data'])
respond_to do |format|
  format.turbo_stream 
end

the log shows the action processing, however without a format pre-defined for the contorller and rendering required objects:

Started POST "/carts/6/update_receipt_number" for 127.0.0.1 at 2023-02-27 08:24:38 +0100
Processing by CartsController#update_receipt_number as */*


  Cart Update (9.6ms)  UPDATE "carts" SET "updated_at" = $1, "receipt_number" = $2 WHERE "carts"."id" = $3
 Rendering carts/update_receipt_number.turbo_stream.erb
  Rendered carts/_receipt_number.html.erb (Duration: 0.8ms | Allocations: 349)
  Rendered carts/update_receipt_number.turbo_stream.erb (Duration: 1.9ms | Allocations: 663)

The turbo_stream.erb response is set as:

<%= turbo_stream.replace :receipt_number do %>
  # replacing the partial with turbo
  <%= render partial: 'carts/receipt_number', locals: {cart: @cart} %>
<% end %>

and the console advises that the HTML is received over the wire XHRPOSThttp://localhost:3000/carts/6/update_receipt_number
[HTTP/1.1 200 OK 357ms] with the following payload

<turbo-stream action="replace" target="receipt_number"><template>
    # replacing the partial with turbo
    <turbo-frame id="receipt_number">
      scan receipt number
<h3>20230224T12423036270004131601843bbaa16909b1b07b2c32414fdea5092de2769aabaa2f610860c827f27fb386d4</h3>
        <form class="button_to" method="post" action="/carts/6/clear_receipt_number"><input type="hidden" name="_method" value="patch" autocomplete="off" /><button class="button warning" type="submit">change</button><input type="hidden" name="authenticity_token" value="DKVdhPK4UfmY_PzvA2amrvh9IejYEnyjHrJQANs0rlj6HHNAZjk3n6zxAgHRg0s27SWwgPZxMXOuLDiqSn6oQw" autocomplete="off" /></form>       
</turbo-frame>
</template></turbo-stream>  

Yet the browser window does not update.

I do not believe the controller's absence of request type is at play, as the response arrives to the browser. So why is the browser not refreshing the turbo_frame? the same was attempted with a div in lieu of the turbo_frame, leading to identical behaviour

What am I missing?

Since you're issuing fetch request yourself, you have to handle the response yourself as well. In other words, you have called a method and now you have to do something with the returned value.

Normally turbo would handle the nitty gritty details, but thankfully, it does expose an api for rendering a stream manually:

fetch("update_receipt_number", {
  method: "post",
  // format: "turbo_stream", // <= i don't think, this is a thing
  body: formData,
  headers: {
    "X-CSRF-Token": document.querySelector("[name='csrf-token']").content,
    // set Accept header to issue a proper TURBO_STREAM request
    "Accept": "text/vnd.turbo-stream.html"
  }
})
.then(response => response.text())
.then(text => Turbo.renderStreamMessage(text));

https://turbo.hotwired.dev/reference/streams#processing-stream-elements

https://developer.mozilla.org/en-US/docs/Web/API/Response/text