Perhaps they are being efficient. Bees often confuse pollen like substances, like corn meal in bird feeders or coal dust, for pollen. maybe these bees find wax on this deep super to be similar enuf to what otherwise would be the source for propolis, so they are gathering it to use like they do propolis.
Now, if you Bee Detectives can stake out the offenders and follow them back to the hive and then witness them using that wax to make comb, then that might be a conclusive observation that bees sometimes do harvest wax from other sources for use in their own hive to build comb.
Let us know if that happens.
What do you call a pollen basket when something else, like propolis or wax, is carried in/on it?