Excerpt not showing in old posts before installing this plugin

Hi,
For the 2nd issue:
In your demo (https://plugins.addonmaster.com/post-grid-with-ajax-filter), we see the Excerpt of posts.
But I do not have the same result. Which Shortcode is missing in the documentation to see the excerpt of the text.
See screen copies: https://drive.google.com/drive/folders/1q6lVlbZk3K-jEUTTdSnyzx_gYVKSTVy-?usp=sharing
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I just realize that new posts have the excerpt displayed.
So I tried to find if I missed an option. I did not find anything different from new and old posts.
But I find that if I write a text inside the excerpt field (bottom right of a post) of an old post, the excerpt become visible. But it’s not a viable solution.
And I find out that when I remove the text from excerpt field (bottom right of a post) of an old post, the excerpt is still displayed. Weird?

Thanks
Alex