When CSS minification is enabled, calc() values will be minified as well, but need to be separated by spaces.
Current behavior: calc(1.5rem+2.5px)
Expected behavior: calc(1.5rem + 2.5px)

Reported by customer: https://secure.helpscout.net/conversation/210133386/16324/?folderId=714999
Original bug in lib fixed in February: mrclay/minify#503