My world hasn't blended well with the new chunks

My world hasn't blended well with the new update. I went to previously rendered chunks and discovered new biomes but there was a very obvious delineation between the old and new. Like, it was the biomes' border is almost in a straight line.

Also, I also experienced massive lags when mining blocksand rendering new chunks. I couldn't see the land in front of me because it's not there and it's not loading.

(Additional side note, my axolotls are still despawning even after I put them in a bucket and later let roam in a pond I made for them.)