I just encountered an impostocube and surprisingly it Didn't drop a gold coin!
I'm not sure if this is new or not but the Impostocube data on the Wiki might need an update. Keep your eyes peeled to confirm this, unfortunately I didn't get a screenshot of the drop but I would have noticed a gold coin standing out from the rest.
Oh yes, I've seen them not drop gold coins for a while now. I'll go update the wiki.