Yup, it's work in progress. I should probably make that clear in the tooltip.
Just FYI, we're considering deprecating the "add option" for dropdown elements. It's really nasty code, and is a legacy from the very early days of Fabrik before the join element was added. Really, if you need options which can be added, you should be using a join element, and storing the data in a table. Having user defined data stored in the Fabrik element metadata, in a format which is not accessible and is subject to change (like, the next time Joomla decide to make sweeping changes to how "attribs" are stored, like they did between 1.0 and 1.5, which caused us a year of pain) is not a good idea.
I'll look at fixing it for the "Advanced format" for now, but you may want to give some thought to your app design, and anticipate that feature getting dropped at some point. Nothing set in stone yet, but we are looking at several legacy issues which cause nothing but pain for us, and take a disproportionate about of time to maintain.
And yeah ... I realized after adding it as a per-element option that it's going to affect every dropdown on the page, as it switches on a global J! "behavior". So we'll probaby have to make that a global (or perhaps per-form) option, and you either use traditional dropdowns. Whether it's per-form or entirely global kind of depends on whether we can constrain / compartmentalize that behavior when you have more than one form on a page.