It is the reading at multiple places in the file at once which is the problem. Rclone has an implicit assumption that there is only one stream reading from a file at once, but rclone can't actually tell from the file system interface.
So what is happening is when the torrent reads from different places in the file then rclone will close the stream and re-open it at the new place. Since there are multiple readers from the file at once it will be doing this all the time.
You could try --vfs-cache-mode full which will download the file first. I'm in the process of finalising a major change to this mode which will mean it does partial downloads and that is probably the mode you want when I've merged it.