In in wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php in WordPress 3.7 to 5.3.0, authenticated users who do not have the rights to publish a post are able to mark posts as sticky or unsticky via the REST API. For example, the contributor role does not have such rights, but this allowed them to bypass that. This has been patched in WordPress 5.3.1, along with all the previous WordPress versions from 3.7 to 5.3 via a minor release.
Created wordpress tracking bugs for this issue:
Affects: epel-6 [bug 1793659]
Affects: epel-7 [bug 1793660]