I created a new Buffet world with the world type "Ocean". In it, I find shipwrecks with chests, as well as underwater ruins. In the chests that contain buried treasure maps in a normal world, there are only empty maps.

I've also tried this in a Buffet world with type "Warm Ocean".

I can't attach anything because the size of the smaller world (zipped) is 14MB. But this is not hard to reproduce. Just create a new Buffet world with type "Ocean" (or I'm assuming one of the sub-types, since I also saw this in a Warm Ocean world), find a shipwreck, look in the map chest, and see an empty map instead of a treasure map.