It seems like lines starting with # (either a nix comment or in a ''double apostrophe'' string, like a shebang for example) reset previous indentation.

{
    # bad indentation on this comment will propagate
    # to the next lines if typing `=i{`
  programs.fish.enable = true;
}

↓ (after typing =i{)

{
    # bad indentation on this comment will propagate
    # to the next lines if typing `=i{`
    programs.fish.enable = true;
  }

This makes it hard to apply the new indentation of a block, when surrounded by new {} for example.