Weird side effects in Nix

flake.nix

{
  inputs.nixpkgs.url = "github:nixos/nixpkgs/a7cc81913bb3cd1ef05ed0ece048b773e1839e51";

  outputs = { self, nixpkgs }: {
    testPathExists =
      let hello = nixpkgs.legacyPackages.x86_64-linux.hello; in
      builtins.pathExists hello;

    testPathExists2 =
      # Output of `nix eval --inputs-from . nixpkgs\#hello.outPath`
      let hello = /nix/store/hrqijlzk6b56ij6g86vdh2b8mkv1i469-hello-2.12.1; in
      builtins.pathExists hello;

    testPathExists3 = self.testPathExists && self.testPathExists2;
    testPathExists4 = self.testPathExists2 && self.testPathExists;
  };
}

trigger.sh

$ nix build .#hello
$ nix eval .\#testPathExists 
true
$ nix eval .\#testPathExists2
false
$ nix eval .\#testPathExists3
true
$ nix eval .\#testPathExists4
false