A cubic envMap seems to skip the first object it's applied to. Subsequent objects referencing the same map are fine. Reordering the objects in the HTML still results in which ever is first being skipped. Reordering in space has no effect.
https://github.com/scottmsinger/aframe/tree/master/tests/env_map_order
Your cubemap tag is missing a close tag. That might be messing with the interpretation of the nextmost tag, causing the issue.
<a-assets>
<a-cubemap id="sky">
...
</a-cubemap> <-- Make sure to add this tag
</a-assets>
If your html/xml editor of choice didn't warn you about this, I'd advice you to switch to a different editor, as it will save you a lot of debugging time. Even quick online tools like JSFiddle highlight XML tag mismatches.
