So, just gonna keep this short...  Upgraded to 1.18, and a certain extreme hills biome that I rather enjoyed and had the intention of building a tunnel through for minecarts, (along with the surrounding plains a forest biomes), has been overwritten by a mesa biome....  A very large Mesa Biome.  Yes, I did visit these chunks, literally the day before the update, so I know what should be here....  But yet, instead of my very nice extreme hills, I find a sweet potato colored mess of a mesa. 

Plus, I also found what may be the shallowest coral reef ever recorded, as behind a village I have spent time in prior to the update, I somehow managed to find a warm ocean biome, completely landlocked, with the most pleasant coral reef having become a part of it upon generation.  Yep, shallowest point was a mere 1 block deep, take that oceanology, I found the world's shallowest coral reef.

Anyways, lucky I made that copy of the world, so hopefully this can be fixed and I can resort to using to previous incarnation of this world and get back to my pixelated existence.

Included you will find a screenshot of me standing on the exact spot where my extreme hills biome should be, so, enjoy?

Rockatoa, Brickticks out!

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Copied from MCPE-149771

Reproduce (requires a Windows PC or iPadOS/iOS device)

• Create a world in the Minecraft version 1.17.41 or earlier, but not before 1.17.0.
• Load all chunks within your render distance, preferably wait a couple of seconds and maybe even a minute so that your chunks load properly in all directions.
• Exit the world and load it into version 1.18.0.
• Notice that some chunks which are a little bit further away from you, have not been touched yet still visible, are replaced by 1.18.0 terrain generation.

Expected Results

• Loaded chunks should not reset when upgrading to 1.18.0.