"Undefined index: override" and "Undefined index: target_id" when saving field configuration

Cannot reproduce in Drupal 10 / PHP 8 (which is the minimum now → )

Reported by a colleague on Drupal 11:

Warning: Undefined array key "override" in entity_reference_override_validate_custom_text() (line 59 of modules/contrib/entity_reference_override/entity_reference_override.module).

entity_reference_override_validate_custom_text()
call_user_func_array() (Line: 281)
Drupal\Core\Form\FormValidator->doValidateForm() (Line: 238)
Drupal\Core\Form\FormValidator->doValidateForm() (Line: 238)
Drupal\Core\Form\FormValidator->doValidateForm() (Line: 238)
Drupal\Core\Form\FormValidator->doValidateForm() (Line: 238)
Drupal\Core\Form\FormValidator->doValidateForm() (Line: 238)
Drupal\Core\Form\FormValidator->doValidateForm() (Line: 238)
Drupal\Core\Form\FormValidator->doValidateForm() (Line: 238)
Drupal\Core\Form\FormValidator->doValidateForm() (Line: 118)
Drupal\Core\Form\FormValidator->validateForm() (Line: 585)
Drupal\Core\Form\FormBuilder->processForm() (Line: 321)
Drupal\Core\Form\FormBuilder->buildForm() (Line: 73)
Drupal\Core\Controller\FormController->getContentResult()
call_user_func_array() (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 593)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 121)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext() (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 183)
Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 53)
Drupal\Core\StackMiddleware\Session->handle() (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 28)
Drupal\Core\StackMiddleware\ContentLength->handle() (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 36)
Drupal\Core\StackMiddleware\AjaxPageState->handle() (Line: 124)
Drupal\cloudflare\CloudFlareMiddleware->handle() (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle() (Line: 709)
Drupal\Core\DrupalKernel->handle() (Line: 19)

With a follow-up note:

Looks like it doesn't throw the error only if 'single line no markup' is used as the custom text format!?