The SiteGround Optimizer plugin works perfectly fine with CSS code that follows the standards for Stylesheets coding. However, no CSS Stylesheet minification plugin works 100% with all the CSS code out there. There could be a variety of plugin/theme combinations which might be applying partial minification or use code which does not follow the full stylesheet standards, in these cases, you can choose to use the other functions of our plugin and pick a different plugin for the minification part.
Despite of this, I will report your findings to our plugin developers and have them test the issue and decide if a fix should be applied in future versions.
Regards,
Plamen.M
Tech Support Supervisor
SiteGround.com