ok now that is interesting. I wonder why? 
edit: because it is supposed to…
I was thinking about going to bed finally but now I am installing tag-icons for the very first time 
edit: I got it to work in the Tag Icons theme component
And yes, you do need to put the icon name in the bottom svg-icon setting - @Moin is correct. In my earlier post, having cocktail there meant it was automatically mapping to martini-glass-citrus.