Dungeon keys won't stack with each other. How do I fix?

As the title says, some of my dungeon keys just won't stack with each other and it's driving me insane. It's eating up so much space in my bank. Sure I could just use them up, but that takes a lot of time and energy, and that's just a temporary fix in the end; I'll get more keys, and the same problem will persist. Is there a fix to this, or should I just accept my descent into madness?