Chunk disappereance under y=0

Im not 100% sure for version but it makes sense the new update caused this.

 

 

I was around in my world and went under y=0 to enter my preveously build tube system. However, i realised it was completely broken! A huge part of the chunks beneath y=0 are gone missing. Not all, but a huge part. All above y=0 are there.  

 

This freaking bug destroyed several hours of playing and building!!!!