Field validation (Constraints) is triggered automatically, only on form validation. As discussed with @fago and @amateescu via IRC, it makes more sense to always validate the entity upon save(), unless explicitly requested not to
* save()'s signature should be changed to public function save($validate = TRUE);
* When an entity is saved via form (i.e. it was already validated), it should be saved without a 2nd validation.
1) Add ER field to node
2) Create node, and notice \Drupal\Core\Entity\EntityFormController::validate calls validations that invoke the Constraints.
3) Try to create the node programmatically, no validation happens.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Surprised i could not find a follow-up but i created one here: ✨ Make easier & document validation of required fields in non-form (programmatic API) contexts Active