ember-cp-validation not executed everytime dependent keys changed

Let me share the problem I stumbled upon longer ago and fixed today.

Problem description:

Validator not executed everytime dependent keys changed.

My custom validator checking uniqueness of custom-meta keys is defined as follows:

import BaseValidator from 'ember-cp-validations/validators/base';
import Ember from 'ember';
const {isEqual} = Ember;

export default BaseValidator.extend({

  /**
   * Validates custom-metas of the {spot} model.
   * The validation leans upon duplicates detection of the 'key' property values.
   * @example:
   * spot.set('customMeta', [{key: 'duplicate'}, {key: 'duplicate'}]);
   * spot.get('validations.attrs.customMeta.isValid') -> false
   * spot.set('customMeta', [{key: 'unique 1'}, {key: 'unique 2'}]);
   * spot.get('validations.attrs.customMeta.isValid') -> true
   * ...skipping rest of the doc...
   */
  validate(value, options, spot) {

    const customMetaKeys = spot.get('customMeta').mapBy('key');

    if(isEqual(customMetaKeys.get('length'), customMetaKeys.uniq().get('length'))){
      return true;
    }

    return this.createErrorMessage('unique-custom-meta-keys', value, options);
  }

});

The validator was executed exactly twice although the dependent keys have changed more often. I thought the problem could come from model-fragments addon or observer that was fired on the same conditions which was related to other feature.

This was my validation declaration:

const Validations = buildValidations({
  customMeta: {
    description: 'Custom-metas',
    validators: [
      validator('unique-custom-meta-key', {
        dependentKeys: ['customMeta.@each.key'],
        debounce: 500
      })
    ]
  }
});

and model definition:

export default Model.extend(Validations, {
  customMeta : fragmentArray('custom-meta')
});