To my Mk1 eyeball they look random, so I'd say the most likely thing is the file has been truncated.
Can you play the mkv to see if it is truncated? Or maybe pass it through ffmpeg?
Taking into account the 32k length multiple of the unencrypted files then this probably happened during the process of upload.
Is it possible the files changed size while they were being uploaded? Rclone should have detected that, but I wonder if that detection isn't working on the FUSE mergerfs mount for some reason.
Great - let me know what happens 