When I compared terrain generation between 1.12.2 and 1.13.pre-1, there were some differences in the biomes. This will cause undesirable chunk discontinuities when upgrading old maps. For example, a Forest biome in 1.12.2 becomes an Ocean biome in 1.13.
Seed: 5116735402577285868
Coordinates: -954 -954
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Working as Intended per MC-125037