Every recording filmed in 1080p48/p50 and p60 i found so far (including my own p4) suffers from a lense distortion effect in the upper left and right corner while panning. Only these modes are effected and recordings in 1080p30,p120, 2,7K and 4K are flawless.
If there was a physical problem with the lense itself this issue should be visible in other modes (4K,1080p30) as well but it isnt. So this must be a software issue in the camera.
If anyone has Videos on Youtube recorded in 1080p 48/50 or 60 feel free to link them. It would help to verify if this is a issue affecting every P4 out there. Panning shots required!
It's a subtle effect - would be interesting to see a stationary video shot of a fine reference grid to see what the distortion effects are. It may be that the camera is using software lens distortion correction but for some reason it isn't working too well in some video modes.
It appears that in the P4, the camera is only using a portion of the sensor when shooting 1080 mode at certain frame rates. At other frame rates and resolution modes, it uses the full area of the sensor. This might have a bearing on the issue.