MK8 Fiesta Volume Problem

Hello

Car- mk8 fiesta st-3 2019, with b&o

Problem:

Radio works flawlessly when engine off. After some seconds the engine is turned on, the radio unit gently starts to lower the volume output. in 1 minute or so, the output is about ~50% of the original volume.

This happens without the actual volume level change, so a 20 volume with engine on equals to 10 volume with engine off.

Happens with all inputs, with or without paired smartphone.

-checked the amp under the seat, all connected and doesn't get hot;

-balance works properly, all speakers work;

-adaptive volume is off;

-reseted the APIM, didnt solve the problem;

-factory reseted the unit, didnt solve the problem;

-dealer says car is ok.

Any idea? At this point i just wish all the speakers and unit to blow up so the dealer can consider it as an actual problem...

Thanks.