User blog comment:Rappy 4187/Technical Update: August 19, 2015/@comment-366087-20150820170047/@comment-1038387-20150820182546

Same thing. On one wiki, I get:
 * My Tools are empty (save for Theme Designer)
 * Date format is set to No Preference.
 * Timezone is set to 0.
 * All "Follow page" and "email me" boxes are ticked, something I didn't have previously.
 * Days and number of edits in RC reset.
 * My landing page is the main page instead of RC.
 * My editor is VisEd instead of Source
 * RC are grouped.

Short version: it seems to have been reset to default, but not on all wikis.

Edit: it's now spreading to other wikis too, which were previously not affected.