Actually you are correct. You will have two video files. the MOV file you are seeing is probably the low resolution thumbnail file in the thumbnail directory. And then you will have the high resolution file. Is that maybe what you are talking about?
Actually, I have been wondering if anyone knows if/how you can change the thumbnail file to record as an MP4 instead of an MOV. The MOV file is viewable but for some reason my video editing software (Premiere Elements) does not like it.
The same is happening to me. Let me add this info:
Previously, using the same version of Go on the same iPad I use for drone flights,
I flew a standard Phantom 4. I used identical settings (choosing a mp4 container) and I would get two files: the .mp4 file and a .thm file with the same base name (ex. DJI_0001.MP4 and DJI_0001.THM) - the .thm is recognized by editing software as a thumbnail file and it contain four tiny jpeg frames arranged in a square grid from the video as a very small preview image. This is a throwback format from decades ago.
Simply switching from the P4 to a new Phantom 4 Pro v2 (everything else the same), now I get two files. The one I specified (for example 1920x1080@30 FPS) .MP4 file plus the same base name .MOV file but alway at only 1280x720@30 fps. I no longer get the .thm file.
I am beginning to think that this is indeed meant as a low resolution/low bitrate preview file. I can’t find any mention (of either result) in any DJI documentation (no surprise).