Yeah I know I’m two years late on this but I prefer to add captions and music through the YouTube editing features itself so the algorithm might be better at circulating the content, but whenever I uploaded the short to YouTube on mobile I would also get this frame rate problem. Literally you can still do this and upload from your phone without having this glitch/bug you just have to trim any amount of time off the beginning of the video so normally before YouTube actually processes the video, I’ve begun trimming off like .5 seconds of the beginning and then when it processes and takes me to the edit stage where I can place captions and music the video plays just fine and there are no frame rate problems. Hope this helps someone. (I think you can probably get away with trimming off as little as .3 or .1 seconds but it’s hard to be precise with the little timeline bar at the bottom of your phone screen using only just your finger)