Overlay for latest flatpak version

Hey all,

I need flatpak 1.15.4 for an application to work properly, so I wanted to install it using an overlay. When rebuilding the system tho I get a different sha256 sum then the one stated on the flatpak github (https://github.com/flatpak/flatpak/releases/tag/1.15.4).

So I get
instead of

My question is if I have done something wrong in my overlay:

nixpkgs = {
  config.allowUnfree = true;
  overlays = [
    (final: prev: {
      flatpak = prev.flatpak.overrideAttrs (old: {
        src = prev.fetchFromGitHub {
          owner = "flatpak";
          repo = "flatpak";
          rev = "1.15.4";
          hash = ""; # Left empty to get sha256sum

As a template I used the example in the wiki (Overlays).

The hash on the release page is for the archive generated by flatpak. We are downloading the auto generated ones and unpack them which gives a different hash.

So everything is correct and I just have to use the hash nix outputs if I understand correctly?