Understandable, the achievement for Dolphins leading you to buried treasure is still there even though one is currently unable to feed raw fish to them, making one unable to acquire the achievement until Aquatic update part two is rolled out.
It currently appears in the list of achievements in 1.4.1 (Switch) and has for at least a couple days on my console.
I have a world that is only 1.4MB, and even after the 1.5.3 hotfix, whenever I try to convert it, it still crashes the game. It is always around 16% or 20%, and just closes out the game with no notifications regarding the crash. It is similar to a few others, it is a redstone superflat world, and the world still works fine on Xbox One edition, so the world is not corrupted.