Select a message and then enable the "unread" filter with the button at the left of the "search article" input/filter box. Something is triggered and all the unread messages currently shown are marked as read. If the "Unread" filter is enabled when no messages are selected, nothing happens. Reproduced with KDE Applications 16.04.3 (5.2.3) and current (2017-02-19) git master.
I set "major" as priority because the bug is a indirect form of data loss (the information about unread messages).
*** This bug has been marked as a duplicate of bug 350731 ***