Allowing elements to be dynamically added to a select box or a set of checkboxes despite Drupal's zealous form validation

Looks like the same problem: New values get lost during form validation when using select2boxes multiple values widget [#3228646] |

My first hint that you definitely could override this core service was chx proposing that one should not be able to: [security weakness] Hardwire FormValidator [#2282089] |

How that actually works: Services and dependency injection in Drupal 8+ | Services and dependency injection | Drupal Wiki guide on