Problem:
The brand new version of ISPConfig 3.1.11 when I add or modify an email transport, no value is displayed anymore on “type”.

Solution:
Ref: https://git.ispconfig.org/ispconfig/ispconfig3/issues/4924
Edit /usr/local/ispconfig/interface/web/mail/mail_transport_edit.php
Change this line:
$app->tpl->setVar($rec, null, true);
to this:
$app->tpl->setVar($rec);
and should work again.