Ordinarily, a form is only validated once, but there are times when a form is resubmitted internally and should be validated again. Setting this to TRUE will force that to happen. This is most likely to occur during Ajax operations.
This property is uncacheable.
Type: bool
protected $must_validate;
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https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Form!FormState.php/property/FormState::must_validate/8.1.x