Doh!
Unlike the behavior from the beginning (a few years ago) through last night, none of the folders beneath All Stories appears in the nav pane. Yesterday, all were there when Unread was selected; today, none.
Folders appear as expected when Saved or All is selected. The numerals to indicate the quantity of stories do not appear when All is selected. (I haven’t used All much, so I don’t know whether they usually appears.)
The stories are accessible if I tap All Stories. So the stories are available locally.
Android and Windows display folders when Unread is selected, so the account behind the interfaces seems OK.
iOS 10.3.1 with 12.7GB available. Rebooting iOS has no obvious effect on the (mis)behavior.