CSS minification removes spaces around operator in calc expressions

A CSS SSX containing the following:

{{p { width: calc(50% + 1px) }}}

is minified to:

p{width:calc(50%+1px)}

Notice the missing spaces around the + operator. This is a syntax error in CSS, the declaration including the calc expression gets entirely discarded by the CSS parser as a consequence.

This also means that ?minify=false does not behave the same as without.