If you look at the event-pins for the Visibility Index of a random domain in the SISTRIX Toolbox, you will notice that the Expanded Sitelinks update might have led to a drastic Visibility loss:
This “loss” of Visibility is primarily due to a change in Google’s SERP layout.
While up to 7 results used to be display for Brand-queries on the first search result page, before the introduction of the Extended Sitelinks update, there is now only one result […] the setback in the SISTRIX Visibility Index can be traced back to a layout update of the Google SERPs. Pages with a strong brand were especially affected by this update.
– SISTRIX Changelog