Advagg with secure pages causes (intermittent) secure content warnings

I suspect this is the 6.x version of this bug

I have securepages and CDN modules installed. I am seeing intermittent cases where secured (https) pages include plain non-ssl CSS urls. In google chrome this results in no styling being applied and a warning in the console, AFAIK firefox now does the same. I tried disabling CDN for admin pages but it still seems to happen when using local css urls so I suspect the issue is within advagg.

It seems that if I do several page refreshes it seems to come good and I usually don't see the problem again for a while. I see it most often in the admin interface (since that's the area I am visiting the most) but I am concerned it's happening in our cart/checkout pages for external visitors as well.

Since the problem seems to go away after a couple of refreshes I am having much difficulty in replicating the problem. It does look very similar to the behaviour reported in the bug mentioned above. I wonder if it happens more after securepages does a redirect, since once I get a good refresh other admin pages seem to be ok, but at times it comes back again. I'll see if I can notice a pattern.