This should be testable with a standard profile D8 installation.
The user--user resource has a user_picture field defined by default.
This field does not show up in the schema definitions ie. [response.definitions] but is visible as a relationship under the paths ie. [response.paths]
Possibly related to: https://www.drupal.org/project/openapi/issues/3000609
Comments
Comment #2
abrammI can confirm this, the issue is caused by the core bug:
https://www.drupal.org/project/drupal/issues/2911651
Applying the core patch fixes this.
Comment #3
abrammComment #4
abramm