I found this post at c-130.net from 2013

There are two fundamental causes, the engine is losing power, or the propeller is demanding too much power (uncontrollable blade angle increase). You isolate which one by watching torque. The moment the the RPM begins to drop, does torque increase or decrease? If torque increases with an RPM decrease, then the propeller is demanding more torque than the engine can put out at that throttle setting, and the engine will die. If torque is dropping as RPM begins to drop, then the engine is putting out less torque than is required to support 25 degrees of propeller blade angle.

If you find the propeller is causing it, begin to look at things that can cause a blade angle increase, such as rigging, valve housing internal linkage interference, NTS linkage bad, torque retaining lug loose, valve housing failure.

If you find the engine is causing it, look for causes that either rob the engine of fuel or lots of air, such as the acceleration bleed air system, blown bleed air duct between check valve and diffuser, kinked fuel lines, clogged fuel system components, fuel control failure, etc. Anything that can affect the efficiency of the engine, such as turbine or compressor damage can also cause these issues. You may also have a malfunction related to the Temp Datum system. If TIT shoots up as the RPM drops, you very likely are losing a massive amount of air. If TIT is dropping as RPM drops, you are likely losing fuel. There really are many causes for this malfunction, so without more information, it is hard to get more specific than that. You must do a full performance run, operationally checking out every aspect of engine and propeller performance, then look at all the engine instrumentation at the time of the malfunction to determine the cause. Little things like the secondary fuel pump pressure light coming on, or the low fuel pressure light illuminating could be important pieces of the puzzle with this one.

however in my case it's all the engines

I have experienced the same. And thanks for your detailed information, but I think there is a major bug in the modeling of the fuel system. This should be repaired.

this is a workaround but you will have to fettle the fuel x-feed as has been already mentioned bugged

Hallo,
I tried serveral options with the Aux an external Tanks. Everytime those tanks are empty this bug doesn’t appear.
Seems to be a bug as so many other things not functioning!
Frustrating