Does not seem to work with revisions

It seems the problem was coming from my own custom wrapper function

public function preserveChanged() {
    if ( property_exists($this->entity, 'changed')) {
      $this->entity->changed->preserve = TRUE;
    }
  }

the property_exists was returning false all the time...
this code works best:

public function preserveChanged() {
    if ($this->entity instanceof FieldableEntityInterface && $this->entity->hasField('changed')) {
      $this->entity->get('changed')->preserve = TRUE;
    }
  }