When defining a fapi #ajax callback inside a vertical tab (==> inside a details element) it never works. This used to work in D7.
You can test this by:
1) Downloading latest dev code from display suite (8.x-2.x)
2) Navigate to "admin/structure/types/manage/article/display"
On the bottom of that screen you see a select box to switch the layout. Switching a layout should trigger an ajax refresh but it doesn't happen. Saving the settings show the selected layout so there isn't a problem with that form it self, just the ajax refresh.
Something changed? I guess #1843220: Convert form AJAX to new AJAX API is responsible...