$currentSection = \Bitrix\Iblock\SectionTable::getById(SECTION_ID)->fetchObject();
$parentRootSection = \Bitrix\Iblock\SectionTable::getList([
'filter' => \Bitrix\Main\ORM\Query\Query::filter()
->where('IBLOCK_ID', $currentSection->getIblockId())
->where('LEFT_MARGIN', '<', $currentSection->getLeftMargin())
->where('RIGHT_MARGIN', '>', $currentSection->getRightMargin())
->where('DEPTH_LEVEL', 1),
])->fetchObject();
var_dump($parentRootSection->getName());