Only 1 message to a single function per interface?

I can’t message an interface that I’ve already implemented a message being sent to, it’s rather strange and I can’t tell if it’s intended behavior or not. See screenshot:

As can be seen, “Mana Value (Message)” is not an option even while the exact same Interface that “Gold Value (Message)” comes from. And they’re made the same way too:

So I really don’t know what I’m doing wrong here.