World conversation destroys half of the world

Thanks Mojang for all your update! I did a search but found nothing of note regarding this bug: when I convert an old world to the new version of minecraft 1.2.3.6 and 1.2.5 Bedrock/better together half of my world is gone! Builds are literally cut in half and replaced with new chunks I converted multiple times to see if a change but-no change! I also converted Amplified world, seemingly with no issue!