Discount type changing automatically

We need a discount on the order subtotal. So I have selected the discount type ' Assigned to Order subtotal '. When we change it to order subtotal It works. After sometime coupons discount type resets to 'Assigned to Order total ' automatically. I have enabled the tax option "Force tax exclusion from order subtotal". Because we need a discount on order amount without tax.  Please help with how to stop restting the discount types automatically.

There is nothing in the system that would automatically change the discount type.  Do you have some external updates to the database?     To troubleshoot this you may need to use SQL and create a trigger in the DB on the Discount table.