Flooded caves and trenches will abruptly get cut off on chunk boundaries near non ocean biomes for absolutely no reason. This did not happen in 1.13.2 or earlier.
Seed: -5613519190629551455
Coordinates: /execute in minecraft:overworld run tp @p -2644 34 -1284
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Relates to MC-146410. Last time I checked this wasn't reported, which is surprising considering how horrible this looks!

affects 20w12a

MC-167213 is completely related to this.

Affects 1.16.4-RC1
Underwater caves no longer exist and have been replaced by aquifiers. Closed as invalid.