Use own python package as dependency in `propagatedBuildInputs`

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1

I have a flake (A) which contains

          packages = {
            pythonPackage = python.pkgs.buildPythonPackage {
              pname = "my-python-lib";
              format = "pyproject";
              version = "1.2";
              propagatedBuildInputs = with python.pkgs; [
                setuptools
              ];
              src = ./.;
            };
          };

And a flake (B) which has (A) as input and the following

          ...
          my-python-lib = inputs.my-python-lib.packages.${system}.pythonPackage;
          ...

          packages = {
            pythonPackage = python.pkgs.buildPythonPackage {
              pname = "my-python-app";
              format = "pyproject";
              version = "1.0";
              propagatedBuildInputs = with python.pkgs; [
                my-python-lib
                setuptools
              ];
              src = ./.;
            };
          };

However, building pythonPackage flake (B) results in an error:

   error: Python version mismatch in 'python3.12-my-python-app-1.0':

   The Python derivation 'python3.12-my-python-app-1.0' depends on a Python derivation
   named 'python3.12-my-python-lib-1.2', but the two derivations use different versions
   of Python:
       'python3.12-my-python-app-1.0' uses /nix/store/kjvgj2n3yn70hmjifg6y0bk9m4rf7jba-python3-3.12.10
       'python3.12-my-python-lib-1.2' uses /nix/store/26yi95240650jxp5dj78xzch70i1kzlz-python3-3.12.9

I think the python version should not important since both projects only contain pure python files. Looking into the derivation of my-python-lib, all python files are there.

Both pyproject.toml have only requires-python = ">=3.12" in it.

Why do the derivations “depend” on the specific python versions? What should I do?