Treasure chests not spawning

I have found four shipwrecks in my exploring the terrain beyond the pre-Bedrock terrain of my world. All four have yielded treasure maps. But only one led to a hidden treasure chest. I tore up the ground at the other three locations but found nothing.

 

What really irritates me is that in an copy of this world, which I use to fly around in creative mode and look at things, there WAS a chest at one of the other three locations. But in my main world it is not there