Cannot handle errors properly using union type

[szado]

Let's say I work with classes like:

readonly class Product
{
    public function __construct(
        public Translation $name,
        public ?Translation $description
    ) {}
}

readonly class Translation
{
    /** @param array<string, string> $values [language => text] **/
    public function __construct(public string $values) {
        if (!$values) { // Any validation here
            throw new CustomException('You need to provide translations!');
        }
    }
}

I have configured ->filterExceptions(...) to process all thrown CustomException objects into proper Valinor errors.

Now, I'm trying to create a Product object from JSON. If I pass an empty array as mainTitle, I receive a MappingError with my custom error for this field as expected. However, if I pass the same invalid value as subtitle, I get a generic CannotResolveTypeFromUnion error that doesn't provide any useful information.

Is there any solution for this problem?