it is in the wireless manual (http://www.mikrotik.com/docs/ros/2.8/in ... ss.content
disconnect-timeout (time; default: 3s) - how long after the disconnect to keep the client in the registration table and keep trying to sending packets
and some information from mikrotik:
The v2.8.19 will be out today, it will contain a fix for disconnect problem. There will be a new parameter which will be documented in the manual.
It is called 'disconnect-timeout' - you can increase the timeout so the AP will still keep
the client in the registration table and try to transmit the packets. If the AP will not
be able to successfully transmit the packets to the client in the the time which is specified
in this new parameter it will remove it from the registration list - disconnect.
(don't know about the powersave handling)