Can't gift Borderlands Handsome Collection on Steam

Other packs are enabled for gifting but the Collection is not. Why?

You can’t buy things you already own. It’s rather frustrating.

That’s not true, for example I own Borderlands 2, and as you can see, “add to cart” is enabled, which leads to the purchase as gift option.

Well I have no idea then. That’s how it’s always been for me.