1.16 RC-1 world opened in 1.15.2 removes everything and replaces with normally generated world

1. Opened any type of 1.15.2 superflat world in 1.16 rc1
2. Used world normally, closed and opened a few times with no problems
3. Close world, close MC, close launcher
4. Open launcher and launch 1.15.2
5. Open world to find yourself in the same coords with the same inventory but the entire world has been replaced with a normally generated world.
6. Close world, close MC, close launcher
7. Open launcher and launch 1.16 rc1
8. Open same world to find the issue unresolved

After trying this with a normal 1.15.2 world I believe that this is a major bug.

I did take a backup before loading into 1.16 rc1 in the first place however I had done quite a lot of work on the world before I tried opening it in 1.15.2 so it would be a shame to lose that