After kmail has started, incoming emails sometimes don't get filtered. Also "Apply filer" on an existing email doesn't do anything. However, after the filter configuration dialog has been opened and closed once, filters work as expected until starting kmail next time. Problem appears with POP3 as well as IMAP accounts. kmail 5.7.1 openSUSE Leap 42.3
Works on a new install of Leap 15.0.