When optimizing a world from 1.12 to pre-release 1, then to pre-release 2, some unloaded chunks will be reset to how they were in the seed. I came across this as I was playing on a world I've had for sometime, I had been playing on it a bit after converting it to pre-release 1 from 1.12.2. When pre-release 2 came out, I decided to switch and optimize the world again due to a bug i was having in pre-release 1. When I loaded back into the world, I went over to an area that unloaded from where I spawned back into the world from, and some chunks here and there had been 'corrupted' and had been reset back to the seed.