Understand python package overlay with flakes

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Hi,
In my nixos flake, I pin the version of a python package (wagtail) to a specific nixpkgs commit with an overlay. My final version works, but I would like to understand why another one didn’t.

For reference, I mostly used : Downgrading or Upgrading Packages | NixOS & Flakes Book, How to override a Python package using callPackage from an overlay - #4 by APCodes, Overriding Python Modules.

The flake.nix:

{
  description = "A flake";
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/24.05";
    # Pinned nixpkgs with Wagtail 5.2.2.
    nixpkgs-6425843.url = "github:nixos/nixpkgs/6425843adacfa1f8a61f3e0fc3e903f333cc1f94";
    disko.url = "github:nix-community/disko";
  };

  outputs = { self, nixpkgs, disko, nixpkgs-6425843 }:
    let
      pkgs = import nixpkgs {
        system = "x86_64-linux";
      };
    in
    {
      nixosConfigurations."machine" = nixpkgs.lib.nixosSystem rec {
        system = "x86_64-linux";
        specialArgs = {
          nixpkgs-6425843 = import nixpkgs-6425843 { inherit system; };
        };
        modules = [
          disko.nixosModules.disko
          ./nixos/configuration.nix
        ];
      };
    };
}

The overlay that works uses callPackage on a raw copy of the wagtail package from the nixpkgs repo. It does not use the pinned nixpkgs flake input :

{ config, pkgs, nixpkgs-6425843, ... }:
{
  nixpkgs.overlays = [ 
    (final: prev: {
      pythonPackagesExtensions = prev.pythonPackagesExtensions ++ [ (pyfinal: pyprev: {
        wagtail = pyfinal.callPackage ./wagtail-6425843.nix {};
      })];
    }) 
  ];
}

Attempt using the pinned nixpkgs input failed. The configuration builds, but wagtail is not added to the site packages (used by uwsgi).

{ config, pkgs, nixpkgs-6425843, ... }:
{
  nixpkgs.overlays = [ 
    (final: prev: {
      pythonPackagesExtensions = prev.pythonPackagesExtensions ++ [ (pyfinal: pyprev: {
        wagtail = nixpkgs-6425843.python3Packages.wagtail;
      })];
    }) 
  ];

Could someone explain why these two overlays are not equivalent ?