The documentation for Ibexa\Contracts\Core\Repository\Validator\ContentValidator::validate() states :

     * @return array Grouped validation errors by field definition and language code, in format:
     *           $returnValue[string|int $fieldDefinitionId][string $languageCode] = $fieldErrors;

However, Ibexa\Core\Repository\Validator\VersionValidator::validate() doesn't do that, instead, it returns with keys $allFieldErrors[$fieldDefinition->identifier][$languageCode]. It uses fieldDefinitionIdentifier instead of Id as it should according to documentation.