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.
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By the way, this has been happening since some point during Beta 1.13 snapshots. I've even seen it happen to my 1.12 creative world - most of the town at spawn would get almost erased and replaced with wilderness.
Now, both of the affected world eventually had customization options added to their level.dat, so there might be that stuff in the chunks, but I removed that stuff from the level.dat (or else I couldn't even convert the world at all), and I would expect that if there's customization data in the chunks, the game would simply ignore that data, not decided to mess up the chunks.