Flarum doesn't natively support nesting tags deeper than the parent, child structure that you see here on discuss now. I made a proof of concept a few years ago where I removed that restriction, but it caused quite some issues back then. With the recent rewrite (where tags are loaded in the background, when needed) it should be possible to implement deeper nested tags more easily.
Whether this ought to be a change on the 1st party tags extension, I'm not sure.
