My Minecraft survival world got replaced by a 1.6.1 world I joined for a few seconds

I joined a 1.6.1 world yesterday for fun just to see how far minecraft has come. I created a world played for 5 minutes got bored. I went back to the original world for survival in 1.16.4, i joined and all of a sudden THAT EXACT WORLD BECAME MY SURVIVAL! I have searched everywhere and nobody has seemed to encounter this bug.