This world was created on the 360 with grass blocks up to Y: 40. When it was converted to Bedrock a year ago, it continued to load the rest of the world at Y: 40 (as expected). Today while going around more of the unexplored areas, the new chunks were loading at Y: 4 (same as a default superflat world). 

Is this a bug or is it a world issue? Is it fixable?

Thank you