Tabbing through the dialog, you can’t proceed from field input to the Add button. You get stuck in the Tag input.
If you try to access the Add button by shift+tabbing through the dialog, you get some weird behavior from the Add button: visually it seems to do nothing, and then if you click Add with the mouse you get the “Added” notice, so apparently it really did do nothing? And also when you use the mouse in that instance, the input Field doesn’t get cleared.