Welcome to Beoworld olegbang.
Yes, this is a problem I have seen before when the powerlink socket is used on the passive amplifier, its a wiring fault on the powerlink socket where if my memory serves me correct pins 1 and 4 are reversed which are yellow and grey I think, but I will need to check the colours as my memory is not what it was in my younger days.
If these are new products and still under warranty you will need to get your dealer to fix this for you. If they are out of warranty and you are happy to resolve this problem yourself, then just reversing the wires defined above will fix the problem. The signal is menn't to shut down the speakers before the TV actually switches off.