i managed to create additional ultracubes in the dimension folding engine when using the recipe cube-ultradense-utility-cube.
when comparing this recipe with other recipes where i don't get additional ultracubes even when productivity modules are used i noticed that in the
recipe cube-ultradense-utility-cube the product "Ultradense utility cube" is not flagged "ignored by productivity" which then causes an additional cube to be created.
the only method to distinguish those cubes from each other is that the real one having its effects shown every few seconds.
if i pick up the wrong one from any inventory it will just poof in my hands and vanish.
