Reproduced the issue on a fresh installed 13.1 by running:

systemd-ask-password --no-tty bla

The log shows that the password request is forwarded correctly to plymouth:

-->--
Jan 23 15:36:54 linux-vi8k systemd[1]: Started Forward Password Requests to
Plymouth.
-- Subject: Unit systemd-ask-password-plymouth.service has finished start-up
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit systemd-ask-password-plymouth.service has finished starting up.
-- 
-- The start-up result is done.
Jan 23 15:36:54 linux-vi8k systemd[1]: Started Forward Password Requests to
Plymouth.
-- Subject: Unit systemd-ask-password-plymouth.service has finished start-up
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit systemd-ask-password-plymouth.service has finished starting up.
-- 
--<--

The problem is that plymouth isn't running:

-->--
linux-vi8k:~ # systemctl status systemd-ask-password-plymouth.service
systemd-ask-password-plymouth.service - Forward Password Requests to Plymouth
   Loaded: loaded
(/usr/lib/systemd/system/systemd-ask-password-plymouth.service; static)
   Active: inactive (dead)
           start condition failed at Fr 2015-01-23 15:36:54 CET; 4min 47s ago
           ConditionPathExists=/run/plymouth/pid was not met
     Docs: http://www.freedesktop.org/wiki/Software/systemd/PasswordAgents

Jan 23 15:36:54 linux-vi8k systemd[1]: Started Forward Password Requests to
Plymouth.
Jan 23 15:36:54 linux-vi8k systemd[1]: Started Forward Password Requests to
Plymouth.
linux-vi8k:~ # 
linux-vi8k:~ # ll /run/plymouth/
total 0
linux-vi8k:~ # 
linux-vi8k:~ # ps aux | grep plymouth | sed '/grep/d'
linux-vi8k:~ # 
--<--

Using kde I saw the same behaviour and detected in the logs that plymouth was
terminated by kdm:

-->--
Jan 23 12:31:46 vm-granita kdm[1774]: plymouth is running
Jan 23 12:31:46 vm-granita kdm[1774]: plymouth is active on VT 7, reusing for
:0
Jan 23 12:31:46 vm-granita kdm[1774]: plymouth should quit after server startup
Jan 23 12:31:47 vm-granita kdm[1774]: Quitting Plymouth with transition
Jan 23 12:31:47 vm-granita kdm[1774]: Is Plymouth still running? no
--<--

Using gnome, I didn't see a message that plymouth gets stopped.
However, obviously it isn't running.

Ismail, could you take a look?