@include flex-item mixin does not work as a single mixin.

[rehamm01]

I tried to use the "@include flex-item" mixin to get the same effect as using these two classes on an element: .fs-offset-left-7 .fs-5.
However, when I tried it did not work. I had to use two flex-item mixins to get the same effect:
@include flex-item(5, 12, null, null, true, null, 0, 0);
@include flex-item(7, 12, 7, left, false);

Is it a bug that this can't be achieved using a single mixin?