5 identical boxes + 1 additional box does not work for Amazon-optimized shipment splits

As @Dominic_Amazonexplained in this post reply, to get Amazon-optimized shipment splits, "the shipments must include at least five identical cartons. Adding an additional shouldn't have an impact."

However, when we did 5 identical boxes + additional boxes, and we were not able to Amaozn-optimized shipment splits. We are able to get Amazon-optimized shipment splits if we have 5 identical boxes + no other boxes.

It also works if we have 6 identical boxes, or 7 identical boxes. However, as soon as we add a box that is not identical, the "Amazon-optimized shipment splits option" becomes grayed out.

Is anyone else having this problem?

We have a massive backlog of unshipped products due to this issue. With 650 kg of products over 75 boxes, we can find a way to get "Amazon-optimized shipment splits" to show up. We're happy to ship to multiple locations and meet Amazon's requirements, but we just can't figure out how.

I hope the requirement isn't that every box we ship must be identical.

Any help from Amazon mods would be greatly appreciated!