To reproduce:
mkdir foo
touch foo/bar
rar a foo.rar foo
file-roller foo.rar

Result:
In File Roller, the 'foo' directory is shown as Type "Unknown" and it's
impossible to enter it by clicking it (though it is possible to browse inside
the archive by typing "/foo" into Location).

Expected result:
The 'foo' directory should be shown as a directory.

Note:
If you use:
  rar a foo.rar foo/bar
then RAR avoids creating a directory entry for 'foo' in the archive (as can be
seen by running 'unrar -t foo.rar'), and then File Roller works fine. This might
give you a hint at fixing the problem.