Chunks gone missing

In the realm I play in, I noticed rather large areas completely missing. 

When I log in the game, it happened a few times, that chunks were missing. 

5 chunks in a row were missing from y-7 to y-23 and almost completely hollowed out. 

The same happened in the end. On the main island a 16x16x16 chunk on the surface was just gone and reappeared later