Something weird is definitely going on there, I got the same results. We’ll need to look into this further, but thanks for bringing to our attention! We’re a bit short staffed this week so I will issue a bug report and we’ll most likely take a look next week. Thanks again!
Quick update here. So, icon sets are only loaded if you’re using a module that has an icon field. In this case, the Tabs module does not have an icon field. Font Awesome is always loaded in our theme, so that’s why those work in your the module. Dashicons are loaded by WP when you’re logged in. In order to use the other icon sets, you’ll need to manually load the stylesheets. Sorry if that wasn’t quite the resolution you were looking for, but we’re actively working on some cool icon set functionality.