I don't know if it is best to ask this question here, or on drones.stackexchange.com, but I think that I am more likely to get a good answer here even if i might have to provide a bit more background.
I experienced this problem on a FPV drone. The drone has an NTSC/PAL camera whose video feed goes into a flight controller (FC) with a built in chip that overlays some important information. The video feed then goes into a video transmitter (VTX) that transmits the signal to the pilot on the ground.
The problem I had was that I powered the camera and VTX directly from the battery via a 5V regulator, while the FC was powered via an electronic speed controller which I assume affects the ground in some way before it reaches the FC.
The problem I had was exactly the same one as this question, and the answer also helped me. I know the problem is caused by a lack of common ground, however what I still don't understand is how a lack of common ground between the FC, camera and VTX can cause the OSD to flicker or even disappear while the video feed from the camera is perfectly fine. I would have thought that the whole video feed would become affected if there was a ground loop.