nix-diff does not show difference in configureFlags

For 2 output paths (shortened to `program1` and `program2`):

```
% nix show-derivation $(nix-store -qd program1) $(nix-store -qd program2) | grep '"configureFlags"'
"configureFlags": "--ld-option=-Wl,--start-group",
"configureFlags": "--enable-executable-static --extra-lib-dirs=/nix/store/qjff0igr8lnajcbfzbr4qqj89vjilg98-ncurses-6.1-20190112/lib --extra-lib-dirs=/nix/store/vw1d2ndp76xdx88j7hdhv2jws44abryk-gmp-6.1.2/lib --extra-lib-dirs=/nix/store/f8kj800is9kv2392wjg66gb3rxvdm17a-libffi-3.2.1/lib",
```

Clearly the two `configureFlags` are different.

But `nix-diff` does not show it:

```
% nix-diff $(nix-store -qd program1) $(nix-store -qd program2)
- /nix/store/xcsm11hy6i4pzry1gr0b4da3ryqm62xv-program-0.1.0.0.drv:{out}
+ /nix/store/bvqxhbmyyyirifn6rcpi6ihkpwzf596l-program-0.1.0.0.drv:{out}
• The set of input names do not match:
- Cabal-2.4.1.0
- gmp-6.1.2
- libffi-3.2.1
- ncurses-6.1-20190112
+ openssl-1.1.1d
+ pkg-config-0.29.2
+ postgresql-11.5
• The input named `mylib-1.2.3` differs
- /nix/store/x5zppd3wmfmbq5hs4j1611rv2nhg21wd-mylib-1.2.3.drv:{out}
+ /nix/store/j7jaf0rzk0a24743rhhkb06aqgzg07sf-mylib-1.2.3.drv:{out}
• The set of input sources do not match:
- /nix/store/01db9xwxmlvqf92mg8jjn54ywp3wmcgg-mylib-src
+ /nix/store/gw9f615d7dg96rch8kazb1fwppas1g9d-mylib-src
• The input named `webserverlib-0.1.0.0` differs
- /nix/store/6slx6ww53q5sa6xxp5b5y5wmmy0mh8qr-webserverlib-0.1.0.0.drv:{out}
+ /nix/store/aidq5xc0dap1mx7pb7m6faqn0r38k27z-webserverlib-0.1.0.0.drv:{out}
• The set of input sources do not match:
- /nix/store/ffcqhwc6jnsq5l4pvxpnagbhvj13f2b6-webserverlib
+ /nix/store/fij4ckb7dasw9mbjmrzxlnmzzs2g573p-webserverlib
```

`configureFlags` is not mentioned in here.

Am I doing something wrong?