On 2019-11-27 (for me around noon), some users on dewiki have been automatically switched to use the enhanced version of the recent changes and the watchlist (the page grouped entry view, not the new edit filters). They had to re-deactivate the feature called "Group changes by page in recent changes and watchlist" on Special:Preferences.
Thread: https://de.wikipedia.org/wiki/Wikipedia:Fragen_zur_Wikipedia#Beobachtungsliste
Was this part of a coordinated action or kind of accidental?
@Toni_Mueller: See the task description for a workaround: "re-deactivate the feature called "Group changes by page in recent changes and watchlist" on Special:Preferences." Did that not work, and/or did you have to do this several times?
This could be T202916: Stop changing my "Group results by page" prefs based on a URL that I've clicked on.
Scenario: there is a public convenience link which contains enhanced=1.
@matej_suchanek @Toni_Mueller that could be the reason. I found a link on dewiki's administartor's noticeboard containing enhanced=1 added on the day when the first complaints popped up. An user also already seemed to be on the right track.
I removed another enhanced=1 link from a category service template, all other occurences I found were older.
This could be closed as duplicate of T202916 IMO.