Minification CSS - icons italic

Hi Balbooa,
On my newest website (not using GB beta), whenever CSS minification is enabled through Gridbox, although Gridbox designer looks OK, but on the live website some of the icons are shown in italics. For example, the social icons and the menu icon - please see images.
Note, the icon list looks ok. Using Chrome by default. The only font used is FontAwesome. No other plugins used.

I've refreshed the page and the cache. When using Firefox for the first time on the website it too see the icons as italics. As soon as CSS minification is turned off, the issue dissappears.

Many thanks,
Marc

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