Same issue with magento 2.2.2.
When you create a cart rule and you select Subtotal as cart attribute condition you are choosing the html option value
It means that the cart rule will use the
quote_address.base_subtotal column to validate the coupon's condition.
If your catalog has the Catalog Prices option (Stores->Configuration->Sales->Tax->Calculation Settings) setted to Excluding Tax the
quote_address.base_subtotal will contain subtotal WITHOUT tax.
Now, digging in the magento core i found that the cart rule condition is validated in
To fix the issue, i changed the code in
$attr = $this->getAttribute();
if ($attr == "base_subtotal") $attr = "base_subtotal_total_incl_tax";
$attributeValue = $model->getData($attr);
quote_address.base_subtotal_total_incl_tax will be used instead of
PS1: i also tried a lot on the web to find a solution and i did not find anything
PS2: i changed and suggested how to fix the issue directly in the core file, but it is a bad pratice, take your precautions :)