Try to set the bit rate to, say, 1.5Mb or 2.0Mb. It worked for both my Samsung S8 and Redmi 5 plus.Hi.
Exact same problem here with a Samsung S4. Pictures are saved correctly. Video recording process seems ok when I launch it on the Tello app on the phone, but it's like no file is saved anywhere when I end the video recording. I'm running on Lineage14 OS.