I just tested after I disabled the plugin, Offload Media, not to copy videos to S3 bucket, and I can see your plugin works great, automatically generating thunmbnails and save them on local my storage.
But not sure what the problem is with uploading video files on S3 bucket.
In some case I saw that the thumbnail was generated in like one out of 50 trials of uploading videos. So I guess it’s related kind of timing issue, maybe.