when you make a world in the newer version and go to the older version your world turns to an ocean biome

so i made a Minecraft world before the new 1.17 update it was in 1.16 than I switched versions to 1.8 to play on a server but accidently went into my 1.16 world that when i loaded in I was drowning and my desert biome was turned into a ocean biome and all my thing were destroyed my Netherite gear was out of my inv and than i went into 1.17 again only to find the same results and than all my item frames and everything I have placed was gone.