Yes, I let the smart phone just default to basic settings. It seems to happen when I am holding the camera over the subject (think like taking a video of your feet). But I ALWAYS HOLD the camera in the landscape orientation. I did not see any earlier suggestion, sorry, new to how this forum works. I was having trouble getting login id/pw maybe missed it.
Thanks
Here is a link to one of the videos that it happened on, two clips total, stitched together with video pad, one gets letterboxed & the next not. My "angle" IIRC was the same pretty much.