After a normal fight for about 20 seconds, about 20 meters high, my AD-8 multicopter (Wookong-M Controller) lost most of its power and went down without until impact.

The flight data recorder (IOSD) shows a significant drop of voltage, from about 15 Volt (4s batteries) to under 8 Volt, diagram attached. With only half of the voltage, of course, there was no way to prevent hard impact.

I'm using two high-quality batteries (4S, 6.75 Ah), which both are less than a year old, in very good condition. Both batteries are connected in parallel, one battery through a current/voltage sensor with telemetry.

What went wrong?

How can it be that two batteries completely give up and the resulting voltage drops to 50% within seconds, with only 5% of the capacity used. After disconnecting the batteries, all 2 x 4 cells show nominal voltages between 4.0 - 4.1V per cell.

I'm pretty sure I checked the voltage of both batteries before flight, but, let's assume I didn't. If one of the two batteries was not charged (only 20% left) while the other battery was fully charged, what would happen? Any correlations with the above scenario?

There should be no voltage difference between two parallel battery or it might cause serious problem. Please fully charge both battery before flight. Since it's not intelligent battery, we can't promise the voltage performance, you need to test the battery by yourself.

It’s important to distinguish voltage drop from voltage loss. You can have both voltage drop and voltage loss in any circuit. You can calculate the voltage drop by using any of several accepted voltage drop formulas. Subtract the resulting number from the measured voltage decrease, and you have voltage loss.