Seems you are right, it's not visible with sunlight. But why does the camera choose different settings when I start via the App? And how can I change these?
EDIT: Setting shutter speed to any manual value allows me to set the speed via the app (to values different from what the dial says, which confuses me). Setting the ISO to Auto via the App sets it automatically to the max of the Auto-Range, no automatic adjustment whatsoever. Setting it to a fixed value works. So: Setting everything (shutter speed, f-stop, iso and focus) you can actually use the WiFi triggered video, except that it is 720p despite the camera being set to 1080p.
Adjusting the shutter speed to slower values (1/30) removed the flickering, which was banding as milandro pointed out.
https://youtu.be/gLN0cig6bKQ
So, I hope for some KaiZen time for the app (and the corresponding part of the firmware) ...
Thanks for the help!
Happy shooting!