Theme issue with Archives plugin?

I've been trying to track down why the Archives Plugin is not working for an inherited Bootstrap theme, and it looks like the theme itself (Bootstrap) might be the issue. A continuous list of months is always displayed, and not just the months containing posts.

This issue can actually be seen on the getgrav.org hosted theme demo site:
http://demo.getgrav.org/bootstrap-skeleton/

As compared with the same content using Antimatter theme:
http://demo.getgrav.org/blog-skeleton

I've tried to figure out the issue myself, but no luck - might this be looked at?

Thanks very much,
Paul