How very problematic.

The UI claims that everything went well and it even goes to show you a diff of the changes you - allegedly - did by rolling back edits, but when you go to the UserBox: page, nothing's changed, and when you look at the userbox in the Special:UserBoxes editing UI, you can furthermore see for yourself that nothing has indeed changed.

The "why?" is kinda obvious: regular MW editing interface has no real knowledge about the data stored on the fantag DB table, which is the only data the Special:UserBoxes UI cares about.

Also, caching probably plays a small role as well (i.e. besides touching the fantag DB table a rollback should also purge the relevant cache key(s) - I presume that after a rollback is complete, something like this should happen:

		$titleString = $some_title_object_representing_the_impacted_userbox_page->getDBkey(); // as per FanBox#__construct, which sets the class' $name property
		$cache = MediaWikiServices::getInstance()->getMainWANObjectCache();
		$key = $cache->makeKey( 'fantag', 'page', $titleString );
		$cache->delete( $key );