Ok. As I said I know that the text is generated from the main description of a single post.

I have text in my post, but this short text it’s not generated from the main description of the single post.

It’s some default short text.

By the way, I found it in a post dashboard under the featured image you should see an excerpt, you will locate that text there, and you can change it there.
I guess that this is the answer to my question.

Regards