So far other than a method of uploading a copy to support through the Beta program I have not gotten any response resolution for this. At what point does the conversion fail for you? Does your Xbox One close Minecraft completely and just put you at your home screen? If you world fails during the converting screen where it is counting up percentages, does it fail at or around the same percentage each time? Do you have any older copies of your world to see if the conversion also fails (maybe at a different percentage)?
John Kemp II
I have a similar world upload issue in a ticket MCPE-26093
https://bugs.mojang.com/browse/MCPE-26093
So far other than a method of uploading a copy to support through the Beta program I have not gotten any response resolution for this. At what point does the conversion fail for you? Does your Xbox One close Minecraft completely and just put you at your home screen? If you world fails during the converting screen where it is counting up percentages, does it fail at or around the same percentage each time? Do you have any older copies of your world to see if the conversion also fails (maybe at a different percentage)?