XBee 3 End Device Extended Sleep Problem

Hello everyone.

I have a problem with extended sleep of an XBee 3 end device.

Configuration can be seen below:
ZS=2
JN=1
EE=1
SM=4
SP=AF0
ST=64
SN=40
SO=6
AP=1
AO=3
D1=2
IR=200
V+=FFFF

Even though I set the necessary registers to achieve extended sleep, device wakes about every 23 seconds (not even 28 seconds that is set with SP register (SP=AF0 which is 28 seconds)).

What is the reason of this situation? What can be done to sleep for entire SP*SN time?

What firmware version are you working with?