The last column of the first row is wrapped to the next line when viewing in Safari, and some other iOS based browsers.
Safari:
This seems to be a bug in Safari's rendering causing the columns to overflow (and therefore wrap) the Bootstrap container (some kind of Webkit rounding error maybe?). Since you are using Bootstrap, you should be able to achieve the desired result without using flex:
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But! Now the problem looking at your example is that you need to keep your product blocks the exact same size or the grid won't keep it's shape. One possible solution is to give .product a fixed height and adjust using media queries.
Here is an example I made that works in Safari and other browsers: http://codepen.io/anon/pen/PZbNMX Additionally, you can use style rules to keep images or description text within a certain size to make things easier to maintain.
Another possible solution is to use a script or jQuery plugin to allow for more dynamic uniform sizing, but I'm not as savvy with that stuff.
I ran into the same issue when trying to combine flex and Bootstrap using Safari, so I hope this helps.