I had been looking for shipwrecks in my single player survival world. I found two treasure maps but when I put them in my hand, nothing shows up on the map. No 'X', no imagery representing water, nothing at all. This happened with both the maps.

Edit I decided to see if this issue extended from that one survival world. I went onto a creative world and flew out to find a shipwreck. Found a new treasure map and the same problem was observed.
It also seems that these bugged treasure maps do stack with one another.