Site is ZC 1.5.5f, PHP version 7.1.2
Tested this on a vanilla install, with only quantity discount module installed.

Store is set to display prices with taxes, and in most cases it does. However, when using a quantity discount, discount amount and sub total are displayed excluding tax.
I traced this back to includes/classes/order.php, line 293 where the current code is

$taxCountryId = $taxZoneId = 0;

Previous versions of zen cart didn't exhibit this issue, and that line in order.php was

$taxCountryId = $taxZoneId = -1;

As suggested by SWGUY, I have modified that line to be

if (DISPLAY_PRICE_WITH_TAX) {
$taxCountryId = $taxZoneId = -1;
} else {
$taxCountryId = $taxZoneId = 0;
}