This is because the
transform creates a new local coordinate system, as per W3C spec:
In the HTML namespace, any value other than
nonefor the transform results in the creation of both a stacking context and a containing block. The object acts as a containing block for fixed positioned descendants.
This means that fixed positioning becomes fixed to the transformed element, rather than the viewport.
There’s not currently a work-around that I’m aware of.
It is also documented on Eric Meyer’s article: Un-fixing Fixed Elements with CSS Transforms.