I've noticed this on 4 different sites now all using different themes that use WordPress Customizer, after using Hummingbird to minify the CSS on the site the customizer changes appear to be lost on the frontend. However they are still there but loaded inline before the minified Hummingbird CSS.
This is obviously a huge problem and it doesn't seem to be a problem with one site. Maybe if a WPMU DEV can make a test site using Divi for example, make a few changes in customizer then minify the CSS using Hummingbird to replicated the issue.
Very simple issue that has devastating effects to any site Hummingbird is installed in using Customizer.