World has changed now has incorrect altitude, missing block graphics, chests are empty and clouds are on ground level

I've been playing this world for several months without issue. I log in to play and noticed that clouds are rendering on what is typically the ground level altitude (64, 65). Double (large) chests appear as a single chest right next to each other - neither contain collected items. 
While on the ground level, the altitude now reads 128, 129. Items that were in the furnace when I last played are no longer there, but when looking at the furnace it appears that something cooking.
Lastly, a polished stone I used to construct a floor now has question mark blocks.
Please see attached images.