Quantity groups benefit + Tiered percent calculator: max tier always applied

[michaelmichael]

I was trying to implement some new bulk discount promotions and noticed something weird with the admin UX for the new promotions gem. I think it's a combination of calculators/benefits that are not intended to be used together (or at least the use case isn't clear to me).

If a user selects the Benefit type "Discount matching line items based on quantity groups" and the calculator type "Tiered Percent based on eligible item quantity", the highest possible tier is automatically applied as long the "Preferred group size" minimum is met.

For example, I set up tiers at 5 (8%) and 10 (10%) with a preferred group size of 5. I expected a customer buying 6 items to get 8%, but they received 10% instead. If the preferred group size is 1, then the highest tier is applied automatically as soon as the item is added to the cart.

Was this intentional, or an oversight? Is there a use case I'm missing here, or is this just an example of benefits and calculators that were not intended to be mixed, and it's causing unexpected results?

Here's an example of the setup I'm using: