I was curious so checked the flying data myself.
Upload and view flight logs from your DJI drones
www.phantomhelp.com
It's always interesting to see the flight data to see what actually happened.
Here's what your data shows ...
There was no GPS location data for the whole, brief flight.
So no horizontal position holding ability at all and no "brakes".
From 0:2.4 to 0:3.2 you gave it full left stick to lift off.
From 0:3.3 to 0:5.4 you gave it full right stick which gave the drone a kick forward, but without GPS it didn't stop when you released the sticks.
From 0:15.6 to 0:18.2 you gave it full backwards right stick, to stop the initial motion but without any ability to brake, the drone would have kept going backwards.
You changed flight mode to Sport at 0:20.1 and from )21.7 till 0:22.9 you gave it half right stick forward, which also pushed the drone and again continued drifting after you released the stick.
At 0:23.9 you pulled the left stick down and stopped the flight.
When flying without GPS, you have to be gentle on the sticks and apply reverse thrust to slow the drone.
Giving it full stick controls will cause alarming motion in a confined area because the drone will continue when you release the sticks.
It's like driving on ice without any brakes.
Your drone did nothing wrong under the circumstances.
There is no hint of anything that could be described as a "flyaway".
Just unexpected motion due to your unfamiliarity with flying without GPS.
If you had just gone straight upwards, rather than making full stick moves in the horizontal plane, the drone would have behaved itself, and quickly acquired satellites and the flight would have been unremarkable.
Or you could have waited longer before lifting off to get GPS and record a home point.