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
Downgrading is not, was never and will never be officially supported.