Notice: Undefined index in user_history_diff_user_history() (line 296 of /var/www/html/docroot/modules/contrib/user_history/user_history.module)

@Rudi Teschner: Thanks for picking this up and for the patch. I have accepted and committed the patch.

Automatically closed - issue fixed for 2 weeks with no activity.

I fear the patch I suggested was just handling the error, but not dealing with the origin of the problem.

I have fields where changes need to be tracked and they are listed in $attached_fields. So far, so good.
But while looping through $attached_fields they are missing from $field_definitions in function user_history_create_user_history and therefore cannot be dealt with

// Get a list of fields attached to the user_history entity.
  $field_definitions = \Drupal::service('entity_field.manager')->getFieldDefinitions('user_history', 'user_history');

The patch above deals with the error. But the problem is that the field_definitions should be made available at this point so that the values can be tracked.

Has this worked for fields added via field api before?

My mistake. I forgot to initialise the user_history database via user_history/update :)