Good question, Deer have already avoided decoys, anything out of the norm will spook a deer. Imagine someone adding something to your bedroom, you'll notice, even become alarmed or inquisitive. Perhaps it's not the form of the target but the target alone that is new/suspicious not to mention the new odors.
Interesting tho, who the heck knows, the deer won't tell us.
I'd imagine enough tho, a leopard resembles a cougar other than coloration, smart animals, remove it and see if things change again.