About a week ago I was flying my p3 at about 300m high straight on top of me. Out of nowhere, the drone completely disconnected from my dji go app. No, I don’t mean “aircraft disconnected”, it literally disconnected like when you turn off your drone but still have the app on. (The disconnected word was in grey). Luckily, it was in line of site so I was able to fly it back. I assumed it was just a random error, so I forgot about it. Today, the same thing happened. I was flying really high up, and it completely disconnected. This seems to only happen when it is at a high altitude, and not when it is far away. What can I do?

This is because directly overhead is the worst place for reception.  Also, you lost video feed, not control signal.  Since you were able to land it.  The Phantom 3 Standard   controller has 1 external, omnidirectional antenna.  That is for the control signal to the aircraft.  It also has 2 flat panel directional antenna built inside the controller.  Those are for video/telemetry..  By flying directly overhead, those antenna could not recieve the signal..  That is what happened.  You still had control of the aircraft, but I believe you were right on the edge of losing that as well, because the control signal external dipole antenna has a doughnut shaped pattern to it.  And doughnuts have a hole in the middle.  Try flying your Phantom a little further away next time.
Hope this helps.

Keep it legal....400 ft.

Agree with 2#. It's not a good place to fly overhead directly.