Why is Wordpress showing a blank excerpt for just one of my posts?

Question

For most of the posts on my Wordpress site, I leave the manual excerpt box empty & let Wordpress generate the excerpt based on the content of my post. Until now, this has been working fine — I get the first 80 words of the post as I would expect based on my settings.

For my most recent post, however, no excerpt shows up with the title of the post on the homepage of my blog. This post itself is quite long with lots of text and images, and the first block in the post is a paragraph with text that is much longer than 80 words.

I'm using Wordpress 5.7.2. I deactivated all my plugins & switched to a Wordpress-provided default theme to make sure no rogue code is getting involved, but still the problem persists.

Answer 1

After a couple hours of debugging, here is what I figured out:

• The unfortunately-named wp_trim_excerpt() does not merely trim the excerpt passed to it, but also assembles an excerpt if a blank string is passed in. (Yes, I see now that this is in the documentation, but I was grep'ing through code trying to trace what was being hooked into get_the_excerpt.)

• The call to excerpt_remove_blocks() is the line within wp_trim_excerpt() which first returned a blank string.

• excerpt_remove_blocks() goes through all the blocks in the post and filters out ones whose names are not on an approved list, called $allowed_inner_blocks, which is stored within the function.

• Although in the editor my post appeared to be a normal post with paragraph and image blocks, all those blocks were secretly part of one big group block with the name core/group, which is not part of $allowed_inner_blocks. Thus, when excerpt_remove_blocks() looped over the blocks, it found one block of an unacceptable type and returned nothing.

• But how would I have known about this group block?! I could find no indication in the block editor that all these blocks were grouped. The block editor worked as it always does, and I could add/delete/move blocks as usual with no issues. I could also create new groups out of existing blocks, but I could not undo this existing group or see any indication of its presence.

• To remove the hidden group, I had to switch into the Code Editor from the Visual Editor. There, I saw the whole text of the post appeared like so:

  <!-- wp:group -->
  <div class="wp-block-group">
     <div class="wp-block-group__inner-container">
  
     ...
  
     </div></div>
  <!-- /wp:group -->
  

• I deleted these lines, updated the post, and voila -- now an excerpt appears as expected on the homepage of my blog.

As to how this group got in there in the first place -- no earthly idea. But I hope this solution now shows up in search to save time for the next poor soul who encounters this situation.