Strokes in symbols not flattening properly with other background elements (CS5 & CS6)

Hi There,

We have a bunch of round button symbols for navigational elements which are constructed in the following way:

1. Circle symbol with a fill and stroke
2. Circle symbol placed into button symbols (e.g. magnifying glass or arrow)
3. Button symbols placed onto page

This allows us to update the background circle symbol and have all the buttons (about 30 of them) update at once.

But when the symbols render on a dark background, they look like badly clear-cut raster images - i.e. the antialiased edges look like they've been flattened on white instead of merging with the background colour like a 24bit png.

Hopefully you can see the kind of bitsy edge in the image below to see what I mean:

I don't recall having this issue in the past - I've fiddled with the stroke settings but still no luck, here are the settings as they currently stand:

Is there a setting that controls this behavior that I might've changed or missed?

Cheers

Ben