Hi Nikou,
I’ve checked raw data from Reach M+ you shared and tried to post-process them. The results seem to be fine.
As far as I know, the Phantom’s camera is installed with a gimbal that allows it to be directed perpendicularly during the flight. That’s might be one of the reasons why you get this offset, as you don’t use a gimbal for Sony camera.
So, It’d be nice to compare your results of flights with different speed values to say something more certain.