This bug is happening to me too. It won't download any updates or new content. This is affecting me on an Xbox One.
This happened to me too, my original spawn was overwritten as well as every chunk on the negative x axis. World was large, 100% explored, with medium biome scale prior o converting.
The bug still occurs on Xbox One in 1.8, has been happening to me since 1.6, My best guess is that it happens because one item gets stuck perpetually downloading (As for how, I have no clue), and that item gets prioritized by the game, locking other items in a perpetual downloading state. An easy fix would (probably) be to stop all downloads upon opening and closing the game. This would cause any perpetual downloads to cease, fixing the issue.