Newly created fields are not being displayed I have a content type "Article" with fields and data that was migrated from a Drupal 7 website.


In the admin/structure/types/manage/article/fields I added a new text field called "Test" with default value some of some HTML added. 
Then, I went to manage form display and disabled that field from the form
Then I went to manage display of the appropriate view mode and enabled that field.
Flushed the cache


Reloading the article page doesn't display the newly added field. I believe it is because there is no value in it, but shouldn't adding a default value to the field pre-populate that field for all previous content belonging to that content type? Or am I missing something here?
Adding a default value doesn’t actually set any value in the field. To do that you must either edit the nodes you want the field on and click save, or write an update hook that sets the field value on all the nodes.

An update hook must be placed in a custom module’s .install file, in your case it would be something like:

function MY_MODULE_update_N() {
  $nodes = \Drupal::entityTypeManager()-&gt;getStorage('node')-&gt;loadByProperties('type' =&gt; 'article');
  foreach ($nodes as $node) {
    $node-&gt;set('field_name', $value);
    $node-&gt;save();
  }
}


As @4k4 mentioned if you have to update too many nodes as you have memory to, you can create batch updates using the &amp;$sandbox, click here for more information