When you use cave world generation, at least with the plains biome, lava does not fill caves completely, causing them to be filled with air. The seed is -473146644224514497
MC-185380
Cave world generation does not fill caves
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Please note that only Java 8 is officially supported - in this case, it does not matter, but it may be for other bugs you are reporting, so please always also test that it happens with Java 8.