This is the successor of [#2367555], which is the issue that several @todos in Drupal core are referring to. Follow-up to [#2263569] Problem/Motivation In [#2263569] forms have been setup to POST ajax requests to their original paths instead of system/ajax. The form is then 'searched for' on the page during the rendering pipeline and once found an Exception is thrown, the rendering aborted and the ajax callback called.

Problem/Motivation Quoting @dawehner from [#2263569-244]: To be clear, using the master request is almost always semantically the wrong thing to do We currently use the master request to populate $form['#action'] in \Drupal\Core\Form\FormBuilder::buildFormAction(). Using getCurrentRequest() caused test failures in \Drupal\system\Tests\Form\RedirectTest, but continuing to use the master request with caused failures in \Drupal\system\Tests\System\AccessDeniedTest