when i change of version on minecraft 1.15.2 to 1.8 all my buildings dissapear

Put the summary of the bug you're having here

What I expected to happen was...: i changed of version in my world to other and then all my buildings dissapear
Describe what you thought should happen here

What actually happened was...:
Describe what happened here

Steps to Reproduce:
1. make a 1.15.2 world 
2.build something, 
3.change to 1.8 version