If you thought Google uses the preview you see in the fetch and render for search purposes, you would be wrong. Google’s John Mueller said on Twitter that the fetch and render preview image is just for you to see what Google sees but it is not used for search.
Here is the tweet:

The preview thumbnail in the console is just to make things easier for you, it’s not used for search. I’d use Fetch & Render to check.
— John ☆.o(≧▽≦)o.☆ (@JohnMu) September 20, 2017
Google did say in the past that the fetch and render is way better than looking at the cached version in Google. But then again, here is John saying it isn’t used in search.
We are still awaiting an update where Google improves the fetch and render tool for more sites.
Forum discussion at Twitter.
This post was scheduled to go live today but was written earlier – I am currently offline today.