It could be a conflict with another plugin. It's impossible to say without looking at the site, though.
When a shortcode is created, it's basically a shortened version of a "call" function. So let's say the shortcode has its own "ID" and another plugin / theme has the same "ID." Which one is the correct one? Unfortunately, WP can't tell the difference so if there is a conflict that could be the reason. Or maybe a rule in your theme code or another plugin is overwriting the shortcode.
So I wouldn't say it's a bug. All you can do is try to isolate the cause of the problem by de-activating plugins / changing themes and seeing if it works, or going to the plugin site and seeing if others are having a similar issue.