1.16 pre-5 backup to 1.15.2 bug

I created a superflat world and build a house.... Two days later, i wanted to play on the 1.15.2 release and i needed to back up and load the map. I did so and it got me into the same world but my construction was not there, i was spawned underground and the world was not superflat. Then i tried going back on the 1.16 pre-5 but the same world was there. I have no idea what happend. I did not change anything to my world's files.

I really need my original world back.

Thank You.