My world didn't blend

I made a world in the snapshot phase for 1.18. I knew creating a world in this phase would be riskable. Therefore my plan was to stay and build in a limited area until 1.18 was released. Today 1.18 released and I could finally fly around and see the rest of the seed. As the images shows, my world hasn't gotten any terrain blending at all. I always played with a render distance of 12 chunks and it looks like after 12 chunks there's a completely different world. Its like my loaded chunks is its own world and there is a different one outside of it. I tried to re-create the world and the seed looks completely different. My guess is that the seed I've got loaded doesn't exist anymore and is replaced by another one. I also don't know if this problem accrued earlier during the snapshot phase because I always stayed near spawn.

I just want this to be clarified and maybe a solution to the problem. I have put a lot of effort in builds and I don't want it to be a border where my world just looks unnatural.