Random Post Link Everywhere WordPress but Multisite

There are gobs of plugins to generate links to random posts, and I’ve coded my own PHP methods as well, but for beginners and especially on WordPress.com I was pleased to discover/find that simply appending ?random on any blog URL (self hosted or WordPress.com does exactly that, makes a link that magically pulls a random post from the site.

http://cogdogblog.com/?random works on my self hosted site as does cogdoglab.wordpress.com/?random for a WP.com one

However on multi-site WordPress sites I manage, this method merely returns the blog home page. This is not a major issue but it seems like inconsistent behavior (one might say it is random behavior??)