I’ve narrowed the bug down to the cstidy module, that is, the code in
wp-content/plugins/jetpack/modules/custom-css/csstidy .
That module is called by function Jetpack_Custom_CSS::minify, defined in
wp-content/plugins/jetpack/modules/custom-css/custom-css.php .
The sass processor in custom-css/custom-css/preprocessors/scss.inc.php produces the correct output, but that output is then munged by csstidy.
As a workaround, I am patching my copy of Jetpack_Custom_CSS::minify to not call csstidy, but instead to return the css directly as passed back after preprocessing.
