This has just happened to me as well. Updated my iOS app to version 1.17 yesterday, loaded my old world and the boundaries have shifted, now 3/4 of my world is invisible. Everything was fine before the update.
Just updated my iOS app to version 1.17 and the borders of my old world have shifted to different chunks. 3/4 of my world is missing unless I change the world type to infinite. This is even happening on the backups that were taken before the update.
My old world is from 2011 and is affected by this bug.