If you are desperate, you could use TextGeometry to label the cube. I have described a method of using TextGeometry to create one-sided 2D text. But it sounds like you wouldn’t need one-sided text, but could use TextGeometry and simply bury the text within each face of the cube.
But the more efficient method would be to (1) use transparent 2D text material either place it over each face (like this); (2) somehow combine the 2D text material with the material for each face and apply the material to each face; or (3) add the text to each texture in Blender.
For #1, I created the text by hand using Blender. For #2, you could combine the materials at the texture level using an html canvas.
I’m sure that others can think of other methods, including how to fix the method you are trying to use. I just thought you might be interested in some of the alternative methods that are available and that I have used.