I'm not entirely sure what's going on here, but something along the
following lines seems to fix it:

diff --git a/lisp/textmodes/css-mode.el b/lisp/textmodes/css-mode.el
index c171bd5..7d364d4 100644
--- a/lisp/textmodes/css-mode.el
+++ b/lisp/textmodes/css-mode.el
@@ -401,11 +401,16 @@
               (cond
                ;; This is a false positive inside a string or comment.
                ((nth 8 (syntax-ppss)) nil)
+               ;; This is a false positive when encountering an
+               ;; interpolated variable.
+               ((eq (char-before (- (point) 1)) ?#) nil)
                ((eq (char-before) ?\})
                 (save-excursion
                   (forward-char -1)
                   (skip-chars-backward " \t")
-                  (unless (bolp) (newline))))
+                  (when (and (not (bolp))
+                             (scss-smie--not-interpolation-p))
+                    (newline))))
                (t
                 (while
                     (progn