yii2: 'required' validator doesn't work on update in REST API

I’m working on a REST API. I extend the default yii\restActiveController and I don’t override any actions. I have this in my model’s rules:

[['status_id', 'priority_id', 'performer_id'], 'required']

But when I try to update a record by sending only {"title":"Some title"}, it gets updated successfully. Other core validators work as expected.