The bug seems to be fixed by the following patch:

diff --git a/lisp/textmodes/css-mode.el b/lisp/textmodes/css-mode.el
index 1759643..e94b11f 100644
--- a/lisp/textmodes/css-mode.el
+++ b/lisp/textmodes/css-mode.el
@@ -450,7 +450,7 @@
 (defun scss-smie--not-interpolation-p ()
   (save-excursion
     (forward-char -1)
-    (or (zerop (skip-chars-backward "[:alnum:]"))
+    (or (zerop (skip-chars-backward "-[:alnum:]"))
         (not (looking-back "#{\\$" (- (point) 3))))))

 ;;;###autoload (add-to-list 'auto-mode-alist '("\\.scss\\'" . 
scss-mode))

I wanted to add a test case for this as well, but I couldn't
understand how the indentation tests work. Are the files in
‘test/indent/’ supposed to exemplify correct indentation? In that
case, the indentation in ‘test/indent/scss-mode.scss’ seems to be
wrong given the default indent offset of 4.