Ash lumps from paint don't stack

After using up several lumps of paint, the ash lumps which were produced won't stack with each other or with normal ash lumps, on the ground, in my hand, or in containers. Since they can't be stacked, they can't be turned into ash blocks, and there is no other way to get rid of them.

Probably some metadata stored on the paint lump isn't being cleared when it's replaced with an ash lump?