ffmpeg automaticall re-scaling. How can I prevent it wihtout having to provide height for 'force_original_aspect_ratio'?

I am using ffmpeg to take screenshots of a video file. It seems some video files end up looking squished because ffmpeg is automatically rescaling them for some reason. I am using a python package to do this, not the CLI tool. The only options I am using are specifying the timestamp to capture and quiet output. I am try to get a lossless shot. I don't know why it is resizing automatically. I tried using the force_original_aspect_ratio option and setting it to -1 but it does not work without specifying the height. Is there any way to prevent this from happening without needing to provide the height? I can do that for my script but it seems unnecessary. It should just keep it as-is.

EDIT: BTW, the video is 720x480. Oddly, the metadata on the video and the screen shot are both 720x480. However I lined them up and the video is clearly wider. VLC takes a screenshot just fine. Someone told me ffmpeg just has issues with SD files. I don't know.