Thanks for the report and proposed fix. I can reproduce this.

• user account has UPDATER role
• $g_due_date_update_threshold = DEVELOPER;
• Test issue with due_date set

$wsdl = 'http://localhost/mantis/api/soap/mantisconnect.php?wsdl';
$s = new SoapClient($wsdl);
$u = 'user';
$p = 'password';
$id = 123;
$issue = $s->mc_issue_get($u, $p, $id);
var_dump($issue->due_date); // string(25) "2024-11-13T13:17:00+00:00"
$issue = $s->mc_issue_update($u, $p, $id, $issue);
$issue = $s->mc_issue_get($u, $p, $id);
var_dump($issue->due_date); // NULL