It should be documented with the "empty_data" option that the data must be passed in view format, in case that differs from model format. For IntegerType, for example, "empty_data" must be set to a string value, not an integer (conversely to what you would intuitively expect):

$builder->add('number', IntegerType::class, [
    'empty_data' => '0', // not: 0
]);

See symfony/symfony#18047