STEPS TO REPRODUCE
1. press "enter" on the login screen or the lock screen
2. fprint dialog invites you to swipe your finger
3. 

OBSERVED RESULT

During swiping my finger, the fprint dialog disappears. I can't see the result
of my swiping and the new invitation. I must type "enter"

EXPECTED RESULT

During swiping my finger, the fprint dialog does not disappear. I can see the
result of my swiping and the new invitation.

The fprint dialog must not disappear until success or returning to password
authentication.

I sent a report to kde team. 

https://bugs.kde.org/show_bug.cgi?id=458424

Here is the answer :

This is an fprint/PAM issue. The fingerprint authentication method has a
timeout and stops being active after a time. That's just the way it works. To
alter this would require changes outside of the scope of KDE, either in fprint,
PAM, or their configuration files.