As discussed in Reddit: https://www.reddit.com/r/pihole/comments/1isvj5g/what_in_the_world_is_this_atrocious_new_static/
The older static lease for DHCP in V5 had a more user-friendly interface, while the new V6 expose a text field for the user write a config file-like text.
While one can argue that the new interface is more powerful, it lacks the ease of use that was a fundamental cornerstone of Pi-Hole.
I suggest reverting to the old GUI, leaving this new one as an "advanced" option for those who want.