The blocks I place in the world disappear if I exit the world and the enter it again.

Put the summary of the bug you're having here
Whenever I place a new block and then exit the world then Re-enter the world again the block(s) disappear.
What I expected to happen was...:
Describe what you thought should happen here
The changes I made in the world would be saved when I enter the world again.
What actually happened was...:
Describe what happened here
The changes I make to the world are not saved.
Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...place a block anywhere
3. ...exit the world and then enter it again