[WooCommerce] Shipping Methods – too many showing

Demo setup at https://wp-test.jezmck.com/

I have two items in my cart; Big and Small.

Big has a shipping class of Heavy, Small has a shipping class of Light.

Shipping Method Flat Rate Expensive is set up as follows:
Cost: 0
Heavy cost: 5
Light: N/A
Calc Type: Per Order

Shipping Method Flat Rate Cheap is set up as follows:
Cost: 0
Heavy cost: N/A
Light: 2
Calc Type: Per Order

The problem is that both Flat Rate Expensive and Flat Rate Cheap **both** appear as options in the checkout.

Surely the Calculation Type of the two shipping methods should mean that any order with both items should result in **only** the Expensive rate being available?