GNU documents that short options without arguments may be combined. uutils accepts these options separately, but rejects the combined forms.

$ printf 'a\nb\n' > a; printf 'a\nc\n' > b
$ diff -qst a b; echo $?

GNU diff 3.12 prints Files a and b differ and exits 1. uutils reports unrecognized option '-qst' and exits 2. diff -ut a b is rejected in the same way, while diff -q -s -t a b works.

#97 and #193 discuss replacing the option parser more generally.

I have a fix ready and will open the PR once #291 is merged, since it builds on that change.