I have a world that has been originally generate in Beta 1.3_01, it's gone all the way through to 1.13 snapshots fine, but when I loaded the 1.13 snapshot converted backup copy of it in 18w45a, half of the world is corrupted - basically, a lot of areas has extensive percentage of chunks regenned and/or repopulated, despite the fact that the chunks were already there. Sometimes, the surface remains largely untouched but anything on y 63 and below gets replaced with ocean.