This is something that may be added in the future, if it’s possible. This is probably a result of how foliage is handled within the editor.
You can however, use a couple of different ways to get the same result.
The easiest way, since you have your cube set up in a blueprint anyway, is to add the single grass mesh to your BP and copy and past it a few times. Make a cluster then copy and paste that cluster a few times to move multiple around to cover your cube face. With this parented by the cube it’ll take on the properties and rotate with it. You can see from my image below how I set this up.
Alternatively you could use the technique we used in the Blueprints Office example from Marketplace and use that for your cubes face for randomization of the grass. [The BP layout is included in this document.][2]
Let me know if you’re still having trouble and I’ll help you further. 
Tim