Ralph,
what you observe is not a bug in CFFI, but a feature of the Common Lisp pathname system. The *FOREIGN-LIBRARY-DIRECTORIES* contains a list of pathname designators. When a string is used as a pathname designator (either by putting a string onto that list or by using the #P reader macro to parse it into a pathname object), the last component of the string is considered to be a file name, not a directory component. It is possible to specify an empty file name in a string used as pathname, though.
To wit:
CL-USER> (describe #P"foo")
#P"foo"
Type: PATHNAME
Class: #<BUILT-IN-CLASS PATHNAME>
TYPE: (PATHNAME . #<CCL::CLASS-WRAPPER PATHNAME #x300040039D2D>)
%PATHNAME-DIRECTORY: NIL
%PATHNAME-NAME: "foo"
%PATHNAME-TYPE: NIL
[...]
CL-USER> (describe #P"foo/")
#P"foo/"
Type: PATHNAME
Class: #<BUILT-IN-CLASS PATHNAME>
TYPE: (PATHNAME . #<CCL::CLASS-WRAPPER PATHNAME #x300040039D2D>)
%PATHNAME-DIRECTORY: (:RELATIVE "foo")
%PATHNAME-NAME: NIL
%PATHNAME-TYPE: NIL
[...]
The CL-FAD library is useful if you need to deal with directory specifications that do not conform to CL's pathname string parsing rules:
CL-USER> (describe (cl-fad:pathname-as-directory #P"foo"))
#P"foo/"
Type: PATHNAME
Class: #<BUILT-IN-CLASS PATHNAME>
TYPE: (PATHNAME . #<CCL::CLASS-WRAPPER PATHNAME #x300040039D2D>)
%PATHNAME-DIRECTORY: (:RELATIVE "foo")
%PATHNAME-NAME: NIL
%PATHNAME-TYPE: NIL
[...]
-Hans