To explore the behaviour of `uiop:directory-files` and `uiop:subdirectory`, come up with a test-case and hopefully eventually put together something portable, I've created a sample directory with the following contents:

$ ls -RF
.:
actual-dir/  link-to-dir@

./actual-dir:
file  link-to-file@
$ 

and run the following code:

     (format t "~{~a~%~}"
      (list
       (uiop:subdirectories "./")
       (uiop:subdirectories (uiop:getcwd))
       (uiop:directory-files "actual-dir/")
       (uiop:directory-files "link-to-dir/")
       (uiop:directory-files "./")
       (uiop:directory-files (uiop:getcwd))))';

(stored in another directory) via `cl` from cl-launch as follows:

for lisp in abcl allegro ccl clisp cmucl ecl lispworks mkcl sbcl; do
  echo "===== $lisp =====";
  env ABCL=abcl-1.3.3 ALLEGRO=alisp CCL=ccl-1.11 CLISP=clisp-2.49 CMUCL=cmucl-21a ECL=ecl-16.1.2 LISPWORKS=lw-console-7.0 MKCL=mkcl-git SBCL=sbcl-1.3.9 \
    cl -l $lisp --require asdf --load ~/foo.lisp
done

This prints the following for me (modulo tiny whitespace changes):

===== abcl =====
(/home/pipping/test/actual-dir/ /home/pipping/test/actual-dir/)
(/home/pipping/test/actual-dir/ /home/pipping/test/actual-dir/)
(/home/pipping/test/actual-dir/file /home/pipping/test/actual-dir/link-to-file)
(/home/pipping/test/link-to-dir/file /home/pipping/test/link-to-dir/link-to-file)
NIL
NIL
===== allegro =====
(/home/pipping/test/link-to-dir/ /home/pipping/test/actual-dir/)
(/home/pipping/test/link-to-dir/ /home/pipping/test/actual-dir/)
(/home/pipping/test/actual-dir/link-to-file /home/pipping/test/actual-dir/file)
(/home/pipping/test/link-to-dir/link-to-file /home/pipping/test/link-to-dir/file)
(/home/pipping/test/link-to-dir)
(/home/pipping/test/link-to-dir)
===== ccl =====
(/home/pipping/test/./actual-dir/)
(/home/pipping/test/actual-dir/)
(/home/pipping/test/actual-dir/file /home/pipping/test/actual-dir/link-to-file)
(/home/pipping/test/link-to-dir/file /home/pipping/test/link-to-dir/link-to-file)
(/home/pipping/test/./link-to-dir)
(/home/pipping/test/link-to-dir)
===== clisp =====
(/home/pipping/test/actual-dir/ /home/pipping/test/actual-dir/)
(/home/pipping/test/actual-dir/ /home/pipping/test/actual-dir/)
(/home/pipping/test/actual-dir/file /home/pipping/test/actual-dir/file)
(/home/pipping/test/actual-dir/file /home/pipping/test/actual-dir/file)
NIL
NIL
===== cmucl =====
(actual-dir/ link-to-dir/)
(/home/pipping/test/actual-dir/ /home/pipping/test/link-to-dir/)
(actual-dir/file actual-dir/link-to-file)
(link-to-dir/file link-to-dir/link-to-file)
NIL
NIL
===== ecl =====
(/home/pipping/test/actual-dir/ /home/pipping/test/actual-dir/)
(/home/pipping/test/actual-dir/ /home/pipping/test/actual-dir/)
(/home/pipping/test/actual-dir/file /home/pipping/test/actual-dir/file)
(/home/pipping/test/actual-dir/file /home/pipping/test/actual-dir/file)
NIL
NIL
===== lispworks =====
(/home/pipping/test/link-to-dir/ /home/pipping/test/actual-dir/)
(/home/pipping/test/link-to-dir/ /home/pipping/test/actual-dir/)
(/home/pipping/test/actual-dir/link-to-file /home/pipping/test/actual-dir/file)
(/home/pipping/test/link-to-dir/link-to-file /home/pipping/test/link-to-dir/file)
NIL
NIL
===== mkcl =====
(/home/pipping/test/actual-dir/ /home/pipping/test/actual-dir/)
(/home/pipping/test/actual-dir/ /home/pipping/test/actual-dir/)
(/home/pipping/test/actual-dir/file /home/pipping/test/actual-dir/link-to-file)
(/home/pipping/test/actual-dir/file /home/pipping/test/actual-dir/link-to-file)
(/home/pipping/test/link-to-dir)
(/home/pipping/test/link-to-dir)
===== sbcl =====
(/home/pipping/test/actual-dir/ /home/pipping/test/link-to-dir/)
(/home/pipping/test/actual-dir/ /home/pipping/test/link-to-dir/)
(/home/pipping/test/actual-dir/file /home/pipping/test/actual-dir/link-to-file)
(/home/pipping/test/link-to-dir/file
 /home/pipping/test/link-to-dir/link-to-file)
NIL
NIL


As you can see,
 - sometimes, a truename will be returned, sometimes not
 - the link to a directory contained in the current directory will sometimes be listed and sometimes be omitted from directory-files for the current directory
 - the link actual-dir/link-to-file will sometimes be dereferenced, so that it shows up as another copy of actual-dir/file in directory-files.