Buy More Save More Breaking In Cart

I've been trying to do a buy more save more purchase since Monday morning, but the promotion breaks and goes away in the cart with some combination of the total number of different items in my cart and the quantity of each of them.  

There's no way to get anywhere close to 100 different comic books in my cart without the discount dropping from all of those comics in the buy more save more.

At some point adding one more unique item will break the promotion.  I can remove one or two items and now increase the quantity of an item already in the cart by like 5-8 and then it breaks again. 

I've gotten up to 160+ items in my cart total with the 75% off working, but it will happen again if I add too many different comics instead of higher quantities of a few comic books. 

Please help! Thank you!

Screenshots below stepping through it. 

Adding one more unique item breaks it:

I can remove one or two items and it works again. 

I can increase the quantity of an item already in the cart by like 5-7 before it breaks again.