Why do post excerpts appear or not in Mastodon ?

Hi !
So, when creating a post with tags on a Piefed instance (feddit.fr), I noticed that the results shown in Mastodon by searching the tags are not always the same: some posts are not shown with an excerpt in Mastodon, when some others do have an excerpt (along with a Read More link).
I would like to know how can one ensure that the excerpt appears, because a blank message with only a link is not great to Mastodon viewers, especially when the posts are short fictions.

Exemples

You can see 2 different behaviours here, where posts appear differently in a Mastodon timeline. This is before clicking on them, and they used the same hashtag (#microfiction):

This one has no excerpt:

(The post is here: https://feddit.fr/c/microfictions/p/261665/nouveau-date )

This one has an excerpt (with markdown formatting even!)

(The post is here: https://piefed.social/c/playground/p/1619496/ligne-de-titre-premiere-ligne-du-message-sans-separation-par-une-ligne-vide )

Can someone point me to precise documentation or tell me if this just need improvement in a future release ? Thanks !

EDIT: I tried to edit one of the posts that weren't properly getting an excerpt, and after publishing my change (one line break at the end), I noticed that following my editing it does NOW have an excerpt when viewed in Mastodon. Strange thin is it doesn't have a link to the post, only a "Read More" link, but the link to the feddit.fr post is linked in the full message after you click Read More:
https://feddit.fr/c/microfictions/p/261665/nouveau-date

EDIT 2: Now the link below the excerpt has arrived, it was just a matter of time.

2 Comments

5PACEBAR@piefed.ca · 3 pts · 234d (1 reply)

Mastodon does not immediately generate link previews and adds a bit of random time to it to avoid sites being brought to their knees should a toot become viral. It can vary from a few minutes to a few hours in my experience.

I can have a look at the code if you want details.

ortaviz@feddit.fr · 1 pts · 234d

Thanks for replying. Do you mean all posts should have an excerpt and the ones who don't are stuck in some bad updating from Mastodon that never really occurs ? If you have spare time to look at it, it would be appreciated yes. I'm willing to file a bug report but obviously I would need to know on which part this failure happens.