Bug or intended?

As we know when you first hit the Checkout Shipping page the delivery date is set to blank.

Then lets say you have multiple shipping options - and lets say it has defaulted to 'Store Pickup' and you change it to 'Delivered' and click Continue (without updating delivery date).

Quite correctly you are warned that you need to enter your delivery date. However your change of shipping option is not remembered and it remains as 'Store Pickup'.

Is this intended functionality or a bug?