It returns a “template” so you can set the width/height you need
You need an alternative solution then.
Twitch’s preview is generally “cached” for a few minutes or so
If you are working in OBS, I use The OBS Websocket plugin and some custom code → https://github.com/Palakis/obs-websocket/blob/4.x-current/docs/generated/protocol.md#takesourcescreenshot