I was playing on the version, 1.8.9 in a multiplayer server.  After playing for 30 minutes I logged off and went to singleplayer.  I forgot I was in 1.8.9 and loaded my 1.19 superflat world by accident. When I joined there was a black screen and I was suffocating.  Then it told me I died, I respawned and the world was a normal creative world now, it wasn't flat.  Also, all of my builds were gone.  I logged back on 1.19 to find the world missing.  This was all on lunar client.  I went on the minecraft launcher and started minecart that way and saw the world.  When I went into it though, the world wasn't superflat and my builds weren't there.  There was also a chunk that was missing in the world and was a big hole in the ground.