If you start with a 100% battery, by the time it's reading 50% the voltage per cell would be reading approx 3.8V.
The cell voltage wouldn't fall to 3.3V until 30% or lower.
This might show why it disconnected and was not able to reconnect.
You flew (green line) up a valley to the right (west) of a hill.
Then climbed to 680 feet as you turned right to fly 330 feet above the hill.
A cross section of the terrain between you and the Phantom when you lost signal shows how close you are to having line of sight blocked by terrain.
The Google Earth topography is approximate but suggests the most likely reason for losing signal was that it was blocked by terrain.
There are many important factors to be aware of in flying a Phantom.
The interaction of battery technology, wind strength at height and the flight dynamics of the Phantom were all involved in this incident.