Steps to reproduce the issue

$q1 = $db->getQuery(true)->select('name, email')->from('customers');
$q2 = $db->getQuery(true)->select('name, email')->from('suppliers');
$q3 = $db->getQuery(true)->select('name, email')->from('shareholders');
$q1->union($q2)->union($q3);
echo $q1->__toString();

Expected result

SELECT name, email FROM customers UNION ( SELECT name, email FROM suppliers) UNION ( SELECT name, email FROM shareholders)

Actual result

SELECT name, email FROM customers

System information (as much as possible)

Joomla 4
MySQL version 5.6.38

Additional comments

Tested with Joomla 3.8.7 with expected result.